This commit enables Cortex-M CPU interrupt sub-priorities
and allows the PendSV interrupt to have a priority different
from the default one. Together these two preprocessor
defines can be used to have PendSV always run as the last interrupt
before returning from the interrupt stack back to the user space.
Running PendSV as the last interrupt before returning to the
user space is recommended by ARM, as it increases efficiency.
Furthermore, that change enhances stability a lot with the
new nRF52 SoftDevice support, currently being worked in
PR #9473.
This commit merely enables sub-priorities and a separate
PendSV priority to be used without changing the default
RIOT behaviour.
4534e9b773