arm: route PendSV to spurious IRQ handler if it is unused

If the PendSV interrupt is not used by Zephyr (this is
the case when we build with single-thread support) we
route the interrupt to z_arm_exc_spurious, instead of
assigning 0 to the vector table entry. This is because
the interrupt is always enabled and always exists, so
it is safer to always get the proper error report, in
case we accidentally pend the PendSV, for any reason.

We also add a comment in the PendSV priority setting,
explaining why it has to be assigned a priority level
even if it is not used.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>