selftests/nolibc: disable ubsan for smash_stack()

smash_stack() intentionally crashes.

Prevent UBSAN from tripping over it.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250419-nolibc-ubsan-v2-6-060b8a016917@weissschuh.net

tools/testing/selftests/nolibc/nolibc-test.c

@@ -1438,6 +1438,7 @@ static int run_vfprintf(int min, int max)
 	return ret;
 }
 
+__attribute__((no_sanitize("undefined")))
 static int smash_stack(void)
 {
 	char buf[100];