tests: arm_runtime_nmi: Make test build on v8m

The ICSR[NMIPENDSET] bit got renamed to ICSR[PENDNMISET] in the v8m
architecture.  So we map SCB_ICSR_PENDNMISET_Msk to
SCB_ICSR_NMIPENDSET_Msk to the tests builds and functions.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>