clockintr_cpu_init: stagger clock interrupts by MAXCPUS

clockintr_cpu_init: stagger clock interrupts by MAXCPUS

During clockintr_cpu_init(), we can't stagger by ncpus because not
every platform has fully incremented it yet.  Instead, stagger by
MAXCPUS.  The resulting intervals are smaller, but are probably still
sufficiently large to avoid aggravating lock contention, even on
platforms where MAXCPUS is large.

While here, don't bother staggering the statclock if it is randomized.

With input from claudio@.