selftests/nolibc: use waitid() over waitpid()

Newer archs like riscv32 don't provide waitpid() anymore.
Switch to waitid() which is available everywhere.

Link: https://lore.kernel.org/r/20241221-nolibc-rv32-v1-2-d9ef6dab7c63@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>