clockintr: add clockintr_stagger()

clockintr: add clockintr_stagger()

clockintr_stagger() formalizes the "staggered start" pattern for
scheduling a common periodic clock interrupt across multiple CPUs.

Right now we implement a staggered start by hand for hardclock(),
statclock(), and schedclock().  I expect we will do it for
profclock(), too.