clockintr: add clockintr_advance_random()

clockintr: add clockintr_advance_random()

Add clockintr_advance_random().  Its sole purpose is to simplify the
implementation of statclock's pseudorandom period.

Ideally, nothing else will use it and we will get rid of it and the
pseudorandom statclock period at some point in the near future.

Suggested by mpi@.

Thread: https://marc.info/?l=openbsd-tech&m=169392340028978&w=2

ok mpi@ mlarkin@