hardclock(9): move setitimer(2) code into itimer_update()

hardclock(9): move setitimer(2) code into itimer_update()

- Move the setitimer(2) code responsible for updating the ITIMER_VIRTUAL
  and ITIMER_PROF timers from hardclock(9) into a new clock interrupt
  routine, itimer_update().  itimer_update() is periodic and runs at the
  same frequency as the hardclock.

  + Revise itimerdecr() to run within itimer_mtx instead of entering
    and leaving it.

- Each schedstate_percpu has its own itimer_update() handle, spc_itimer.
  A new scheduler flag, SPCF_ITIMER, indicates whether spc_itimer was
  started during the last mi_switch() and needs to be stopped during the
  next mi_switch() or sched_exit().

- A new per-process flag, PS_ITIMER, indicates whether ITIMER_VIRTUAL
  and/or ITIMER_PROF are running.  Checking the flag is easier than
  entering itimer_mtx to check process.ps_timer[].  The flag is set
  and cleared in a new helper function, process_reset_itimer_flag().

- In setitimer(), call need_resched() when the state of ITIMER_VIRTUAL
  or ITIMER_PROF is changed to force an mi_switch() and update
  spc_itimer.

claudio@ notes that ITIMER_PROF could be implemented as a high-res
timer using the thread's execution time as a guide for when to
interrupt the process and assert SIGPROF.  This would probably work
really well in single-threaded processes.  ITIMER_VIRTUAL would be
more difficult to make high-res, though, as you need to exclude time
spent in the kernel.

Tested on powerpc64 by gkoehler@.  With input from claudio@.

Thread: https://marc.info/?l=openbsd-tech&m=169038818517101&w=2

ok claudio@