Timers may have specific CPU affinity requirements in that their
backing clock device might not beat on all available CPUs, but only on
a subset of them.

The CPU affinity of every timer bound to a particular thread has to be
tracked each time such timer is started, so that no core timer is
queued to a per-CPU list which won't receive any event from the
backing clock device.

Such tracking was missing for timerfd and POSIX timers, along with
internal timers running the sporadic scheduling policy.

At this chance, the timer affinity code was cleaned up by folding all
the affinity selection logic into a single call,
i.e. xntimer_set_affinity().