tools/nolibc: add testcase for fork()/waitpid()

On s390 the arguments to clone() which is used by fork() are different
than other archs.
Make sure everything works correctly.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>