core: Allow the virtualization of NVIC functions

core: Allow the virtualization of NVIC functions

Previously, we only allowed the overriding of the NVIC defines on select
architectures. This meant that the CMSIS_VECTAB_VIRTUAL and
CMSIS_NVIC_VIRTUAL features were not practically useful on any of the
other architectures.

On all architectures, allow overriding the NVIC defines by including a
standardly named "cmsis_vectab_virtual.h" and "cmsis_nvic_virtual.h"
based on whether virtualization is desired (via CMSIS_VECTAB_VIRTUAL and
CMSIS_NVIC_VIRTUAL).