tools/nolibc: tests: use volatile to force stack smashing

Use a volatile pointer to write outside the buffer so the compiler can't
optimize it away.

Reported-by: Alexey Dobriyan <adobriyan@gmail.com>
Link: https://lore.kernel.org/lkml/c0584807-511c-4496-b062-1263ea38f349@p183/


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>