introduce function pointers once set up to the respective implementation
for reading the (S)TICK and writing the (S)STICK_COMPARE registers as a
compromise between duplicating code and selecting between different
implementations during execution over and over again, similar to what is
done elsewhere in the MD in order to support different CPU models that
won't ever change at runtime.
- In the remaining tick interrupt handler further push down disabling of
interrupts to the periodic case as it isn't necessary here in one-shot
mode at all.
36c7255a81