clockintr, statclock: eliminate clockintr_statclock() wrapper

clockintr, statclock: eliminate clockintr_statclock() wrapper

- Move remaining statclock variables from kern_clockintr.c to
  kern_clock.c.  Move statclock variable initialization from
  clockintr_init() into initclocks().

- Change statclock() prototype to make it a legal clockintr
  callback function and establish the handle with statclock()
  instead clockintr_statclock().

- Merge the contents of clockintr_statclock() into statclock().
  statclock() can now reschedule itself and handles multiple
  expirations transparently.

- Make statclock_avg visible from sys/systm.h so that clockintr_cpu_init()
  can use it to advance the statclock across suspend/hibernate.

Thread: https://marc.info/?l=openbsd-tech&m=169428749720476&w=2