Start file for freestanding c++

stubstart.S

// https://blogs.oracle.com/linux/post/hello-from-a-libc-free-world-part-1
// entry code for compiling with `-nostdlib` :
// `gcc -nostdlib -c -fno-no-stack-protector -fno-exceptions -fno-asynchronous-unwind-tables hello.c` // compile without linking
// `gcc -nostdlib -c stubstart.S` // compile the entry code
// `ld stubstart.o hello.o -o hello` // link the entry code and the compiled code
.global _start //global entry point for the program.
_start: call main //enter the main function
        movl $1, %eax // put 1, the syscall number for SYS_exit
        xorl %ebx, %ebx // put 0, the exit code for success (argument to SYS_exit)
        int $0x80 // call the kernel interrupt to exit