selftests/nolibc: add chmod_argv0 test

argv0 is readable and chmodable, let's use it for chmod test, but a safe
umask should be used, the readable and executable modes should be
reserved.

Reviewed-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Zhangjin Wu <falcon@tinylab.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>