selftests/nolibc: add armthumb configuration

While nolibc does support ARM Thumb instructions,
that support was not tested specifically.

Add a new test configuration for it.

Tested-by: Willy Tarreau <w@1wt.eu>
Link: https://lore.kernel.org/r/20250301-nolibc-armthumb-v1-2-d1f04abb5f6d@weissschuh.net


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