selftests/nolibc: explicitly enable ARM mode

The default could also be -mthumb.

Explicitly use -marm to keep everything predictable.

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


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