selftests/nolibc: add x32 test configuration

Nolibc supports the x32 ABI on x86.
Add a testcase to make sure the support stays functional.

QEMU user does not have support for x32, so skip the test there.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250712-nolibc-x32-v1-2-6d81cb798710@weissschuh.net