kselftest/arm64: tpidr2: Switch to waitpid() over wait4()

wait4() is deprecated, non-standard and about to be removed from nolibc.

Switch to the equivalent waitpid() call.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Reviewed-by: Mark Brown <broonie@kernel.org>
Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Link: https://lore.kernel.org/r/20250821-nolibc-enosys-v1-6-4b63f2caaa89@weissschuh.net