tools/nolibc: drop wait4() support

Not all architectures implement the wait4() syscall. It can be
implemented in terms of the waitid() syscall, but that would require
some rework of the other wait-related functions in wait.h.

As wait4() is non-standard and deprecated, remove it.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250821-nolibc-enosys-v1-7-4b63f2caaa89@weissschuh.net