drift in order to achieve a more stable clock as the tick intervals may
vary in the first place. In fact I haven't seen this code kick in when
in oneshot-mode so just skip it in that case.
- There's no need to explicitly stop the (S)TICK counter in oneshot-mode
with every tick as it just won't trigger again with the (S)TICK compare
register set to a value in the past (with a wrap-around once every ~195
years of uptime at 1.5 GHz this isn't something we have to worry about
in practice).
- Given that we'll disable interrupts completely anyway there's no
need to enter critical sections.
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