Yup, it'd make it (also using SYS_exit instead of SYS_exit_group) compilable under various *BSD OSes. Doesn't work for some reason under NetBSD due to some linking problem producing invalid binary for the platform.

  $ gcc -nostartfiles a.c -o a -Wl,--entry=_start -static
  $ ./a
  -bash: ./a: cannot execute binary file: Exec format error
  $ file a
  a: ELF 64-bit LSB executable, x86-64, version 1 (SYSV), statically linked, not stripped\

But the final binary is impressively small, and it seem to contain all relevant code (body of _start and syscall funcs)

  $ strip -s a; ls -l a
  -rwxr-xr-x  1 user  users  2688 Jan  5 07:53 a

Works under FreeBSD though:

  $ clang -nostartfiles a.c -o a -static
  $ ./a
  Hello world!
  $ strip -s a; ls -l a
  -rwxr-xr-x  1 user  user  9216 Jan  5 07:59 a