tools/nolibc: fix incorrect truncation of exit code

Ammar Faizi reported that our exit code handling is wrong. We truncate
it to the lowest 8 bits but the syscall itself is expected to take a
regular 32-bit signed integer, not an unsigned char. It's the kernel
that later truncates it to the lowest 8 bits. The difference is visible
in strace, where the program below used to show exit(255) instead of
exit(-1):

  int main(void)
  {
        return -1;
  }

This patch applies the fix to all archs. x86_64, i386, arm64, armv7 and
mips were all tested and confirmed to work fine now. Risc-v was not
tested but the change is trivial and exactly the same as for other archs.

Reported-by: Ammar Faizi <ammar.faizi@students.amikom.ac.id>
Cc: stable@vger.kernel.org
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>