selftests/nolibc: rename s390 to s390x

Support for 32-bit s390 is about to be added.
As "s39032" would look horrible, use the another naming scheme.
32-bit s390 is "s390" and 64-bit s390 is "s390x",
similar to how it is handled in various toolchain components.

Acked-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250206-nolibc-s390-v2-1-991ad97e3d58@weissschuh.net


Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>