tests: kernel: mem_protect: Handle thumb instructions for Cortex-R

Check for COMPILER_ISA_THUMB2 rather than CPU_CORTEX_M when determining
if special handling is required for thumb instructions.

This prevents false negative results on Cortex-R devices that may
generate a fault (test pass) even if the MMU/MPU is not configured
correctly.

Signed-off-by: Grant Ramsay <gramsay@enphaseenergy.com>