arch: arm: cortex_m: restrict usage of atomic_operations_c for Cortex-m0/+

Armv8-m baseline support various instruction carrying exclusive-monitor and
acquire-release semantic. By adding this guard we let armv8-m.baseline
fall-back to arch defined or compiler built-in support for atomic
operations.

Signed-off-by: Wilfried Chauveau <wilfried.chauveau@arm.com>