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

Author: t-8ch

tools/testing/selftests/arm64/abi/tpidr2.c

@@ -182,16 +182,16 @@ static int write_clone_read(void)
 	}
 
 	for (;;) {
-		waiting = wait4(ret, &status, __WCLONE, NULL);
+		waiting = waitpid(ret, &status, __WCLONE);
 
 		if (waiting < 0) {
 			if (errno == EINTR)
 				continue;
-			ksft_print_msg("wait4() failed: %d\n", errno);
+			ksft_print_msg("waitpid() failed: %d\n", errno);
 			return 0;
 		}
 		if (waiting != ret) {
-			ksft_print_msg("wait4() returned wrong PID %d\n",
+			ksft_print_msg("waitpid() returned wrong PID %d\n",
 				       waiting);
 			return 0;
 		}