Why the libc?

gistfile1.txt

~/.platformio/packages/toolchain-gccarmnoneeabi/bin/arm-none-eabi-g++ -o .pioenvs/test/firmware.elf -mfloat-abi=hard -mfpu=fpv4-sp-d16 -Os -mthumb -Wl,--gc-sections -Wl,--check-sections -Wl,--unresolved-symbols=report-all -Wl,--warn-common -Wl,--warn-section-align -mcpu=cortex-m4 -Wl,-Tlib/ASF4/src/device/same51j20a_flash.ld .pioenvs/test/libbf5/ASF4/device/startup_same51.o .pioenvs/test/libbf5/ASF4/device/system_same51.o .pioenvs/test/libbf5/ASF4/hal/src/hal_adc_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_atomic.o .pioenvs/test/libbf5/ASF4/hal/src/hal_cache.o .pioenvs/test/libbf5/ASF4/hal/src/hal_delay.o .pioenvs/test/libbf5/ASF4/hal/src/hal_ext_irq.o .pioenvs/test/libbf5/ASF4/hal/src/hal_gpio.o .pioenvs/test/libbf5/ASF4/hal/src/hal_i2c_m_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_init.o .pioenvs/test/libbf5/ASF4/hal/src/hal_io.o .pioenvs/test/libbf5/ASF4/hal/src/hal_mci_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_sleep.o .pioenvs/test/libbf5/ASF4/hal/src/hal_spi_m_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_usart_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_usb_device.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_assert.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_event.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_list.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_syscalls.o .pioenvs/test/libbf5/ASF4/hpl/adc/hpl_adc.o .pioenvs/test/libbf5/ASF4/hpl/cmcc/hpl_cmcc.o .pioenvs/test/libbf5/ASF4/hpl/core/hpl_core_m4.o .pioenvs/test/libbf5/ASF4/hpl/core/hpl_init.o .pioenvs/test/libbf5/ASF4/hpl/dmac/hpl_dmac.o .pioenvs/test/libbf5/ASF4/hpl/eic/hpl_eic.o .pioenvs/test/libbf5/ASF4/hpl/gclk/hpl_gclk.o .pioenvs/test/libbf5/ASF4/hpl/mclk/hpl_mclk.o .pioenvs/test/libbf5/ASF4/hpl/osc32kctrl/hpl_osc32kctrl.o .pioenvs/test/libbf5/ASF4/hpl/oscctrl/hpl_oscctrl.o .pioenvs/test/libbf5/ASF4/hpl/pm/hpl_pm.o .pioenvs/test/libbf5/ASF4/hpl/ramecc/hpl_ramecc.o .pioenvs/test/libbf5/ASF4/hpl/sdhc/hpl_sdhc.o .pioenvs/test/libbf5/ASF4/hpl/sercom/hpl_sercom.o .pioenvs/test/libbf5/ASF4/hpl/usb/hpl_usb.o .pioenvs/test/libbf5/ASF4/sd_mmc/sd_mmc.o .pioenvs/test/libbf5/ASF4/usb/class/cdc/device/cdcdf_acm.o .pioenvs/test/libbf5/ASF4/usb/device/usbdc.o .pioenvs/test/libbf5/ASF4/usb/usb_protocol.o .pioenvs/test/src/atmel_start.o .pioenvs/test/src/driver_init.o .pioenvs/test/src/main.o .pioenvs/test/src/sd_mmc_start.o .pioenvs/test/src/sdtester.o -L.pioenvs/test -Wl,--start-group .pioenvs/test/lib149/libUSBPrintf.a .pioenvs/test/lib46c/libArduinoJson_ID64.a .pioenvs/test/lib7c2/libADXL375.a .pioenvs/test/libe81/libBMI088.a -larm_cortexM4lf_math -lm -Wl,--end-group --verbose
Using built-in specs.
COLLECT_GCC=/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/arm-none-eabi-g++
COLLECT_LTO_WRAPPER=/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/lto-wrapper
Target: arm-none-eabi
Configured with: /home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/src/gcc/configure --target=arm-none-eabi --prefix=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native --libexecdir=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/lib --infodir=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/share/doc/gcc-arm-none-eabi/info --mandir=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/share/doc/gcc-arm-none-eabi/man --htmldir=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/share/doc/gcc-arm-none-eabi/html --pdfdir=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/share/doc/gcc-arm-none-eabi/pdf --enable-languages=c,c++ --enable-plugins --disable-decimal-float --disable-libffi --disable-libgomp --disable-libmudflap --disable-libquadmath --disable-libssp --disable-libstdcxx-pch --disable-nls --disable-shared --disable-threads --disable-tls --with-gnu-as --with-gnu-ld --with-newlib --with-headers=yes --with-python-dir=share/gcc-arm-none-eabi --with-sysroot=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/install-native/arm-none-eabi --build=x86_64-linux-gnu --host=x86_64-linux-gnu --with-gmp=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-mpfr=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-mpc=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-isl=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-cloog=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-libelf=/home/build/toolchain/gcc-arm-none-eabi-4_8-2014q3-20140805/build-native/host-libs/usr --with-host-libstdcxx='-static-libgcc -Wl,-Bstatic,-lstdc++,-Bdynamic -lm' --with-pkgversion='GNU Tools for ARM Embedded Processors' --with-multilib-list=armv6-m,armv7-m,armv7e-m,armv7-r
Thread model: single
gcc version 4.8.4 20140725 (release) [ARM/embedded-4_8-branch revision 213147] (GNU Tools for ARM Embedded Processors) 
COMPILER_PATH=/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/bin/
LIBRARY_PATH=/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/lib/armv7e-m/fpu/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../arm-none-eabi/lib/armv7e-m/fpu/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/lib/:/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../arm-none-eabi/lib/
COLLECT_GCC_OPTIONS='-o' '.pioenvs/test/firmware.elf' '-mfloat-abi=hard' '-mfpu=fpv4-sp-d16' '-Os' '-mthumb' '-mcpu=cortex-m4' '-L.pioenvs/test' '-v'
 /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/collect2 --sysroot=/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../arm-none-eabi -X -o .pioenvs/test/firmware.elf /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu/crti.o /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu/crtbegin.o /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/lib/armv7e-m/fpu/crt0.o -L.pioenvs/test -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/lib/armv7e-m/fpu -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../arm-none-eabi/lib/armv7e-m/fpu -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4 -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/lib -L/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../arm-none-eabi/lib --gc-sections --check-sections --unresolved-symbols=report-all --warn-common --warn-section-align -Tlib/ASF4/src/device/same51j20a_flash.ld .pioenvs/test/libbf5/ASF4/device/startup_same51.o .pioenvs/test/libbf5/ASF4/device/system_same51.o .pioenvs/test/libbf5/ASF4/hal/src/hal_adc_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_atomic.o .pioenvs/test/libbf5/ASF4/hal/src/hal_cache.o .pioenvs/test/libbf5/ASF4/hal/src/hal_delay.o .pioenvs/test/libbf5/ASF4/hal/src/hal_ext_irq.o .pioenvs/test/libbf5/ASF4/hal/src/hal_gpio.o .pioenvs/test/libbf5/ASF4/hal/src/hal_i2c_m_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_init.o .pioenvs/test/libbf5/ASF4/hal/src/hal_io.o .pioenvs/test/libbf5/ASF4/hal/src/hal_mci_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_sleep.o .pioenvs/test/libbf5/ASF4/hal/src/hal_spi_m_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_usart_sync.o .pioenvs/test/libbf5/ASF4/hal/src/hal_usb_device.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_assert.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_event.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_list.o .pioenvs/test/libbf5/ASF4/hal/utils/src/utils_syscalls.o .pioenvs/test/libbf5/ASF4/hpl/adc/hpl_adc.o .pioenvs/test/libbf5/ASF4/hpl/cmcc/hpl_cmcc.o .pioenvs/test/libbf5/ASF4/hpl/core/hpl_core_m4.o .pioenvs/test/libbf5/ASF4/hpl/core/hpl_init.o .pioenvs/test/libbf5/ASF4/hpl/dmac/hpl_dmac.o .pioenvs/test/libbf5/ASF4/hpl/eic/hpl_eic.o .pioenvs/test/libbf5/ASF4/hpl/gclk/hpl_gclk.o .pioenvs/test/libbf5/ASF4/hpl/mclk/hpl_mclk.o .pioenvs/test/libbf5/ASF4/hpl/osc32kctrl/hpl_osc32kctrl.o .pioenvs/test/libbf5/ASF4/hpl/oscctrl/hpl_oscctrl.o .pioenvs/test/libbf5/ASF4/hpl/pm/hpl_pm.o .pioenvs/test/libbf5/ASF4/hpl/ramecc/hpl_ramecc.o .pioenvs/test/libbf5/ASF4/hpl/sdhc/hpl_sdhc.o .pioenvs/test/libbf5/ASF4/hpl/sercom/hpl_sercom.o .pioenvs/test/libbf5/ASF4/hpl/usb/hpl_usb.o .pioenvs/test/libbf5/ASF4/sd_mmc/sd_mmc.o .pioenvs/test/libbf5/ASF4/usb/class/cdc/device/cdcdf_acm.o .pioenvs/test/libbf5/ASF4/usb/device/usbdc.o .pioenvs/test/libbf5/ASF4/usb/usb_protocol.o .pioenvs/test/src/atmel_start.o .pioenvs/test/src/driver_init.o .pioenvs/test/src/main.o .pioenvs/test/src/sd_mmc_start.o .pioenvs/test/src/sdtester.o --start-group .pioenvs/test/lib149/libUSBPrintf.a .pioenvs/test/lib46c/libArduinoJson_ID64.a .pioenvs/test/lib7c2/libADXL375.a .pioenvs/test/libe81/libBMI088.a -larm_cortexM4lf_math --end-group -lstdc++ -lm --start-group -lgcc -lc --end-group /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu/crtend.o /home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/armv7e-m/fpu/crtn.o
/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/bin/ld: warning: changing start of section .bss by 4 bytes
/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/bin/ld: warning: changing start of section .bss by 4 bytes
/home/timv/.platformio/packages/toolchain-gccarmnoneeabi/bin/../lib/gcc/arm-none-eabi/4.8.4/../../../../arm-none-eabi/bin/ld: warning: changing start of section .bss by 4 bytes