How to set %gs base on Darwin

set_gs_darwin.s

# _seems_ to make %gs base point to address in %rdi
# on x86-64 Darwin
#
# (what would be amd64_set_gsbase on FreeBSD, or
# arch_prctl with ARCH_SET_GS on Linux)
#
# Could find exacty zero documentation on this!

movl $0x3000003, %eax;
syscall;