statclock: move profil(2), GPROF code to profclock(), gmonclock()

statclock: move profil(2), GPROF code to profclock(), gmonclock()

This patch isolates profil(2) and GPROF from statclock().  Currently,
statclock() implements both profil(2) and GPROF through a complex
mechanism involving both platform code (setstatclockrate) and the
scheduler (pscnt, psdiv, and psratio).  We have a machine-independent
interface to the clock interrupt hardware now, so we no longer need to
do it this way.

- Move profil(2)-specific code from statclock() to a new clock
  interrupt callback, profclock(), in subr_prof.c.  Each
  schedstate_percpu has its own profclock handle.  The profclock is
  enabled/disabled for a given CPU when it is needed by the running
  thread during mi_switch() and sched_exit().

- Move GPROF-specific code from statclock() to a new clock interrupt
  callback, gmonclock(), in subr_prof.c.  Where available, each cpu_info
  has its own gmonclock handle .  The gmonclock is enabled/disabled for
  a given CPU via sysctl(2) in prof_state_toggle().

- Both profclock() and gmonclock() have a fixed period, profclock_period,
  that is initialized during initclocks().

- Export clockintr_advance(), clockintr_cancel(), clockintr_establish(),
  and clockintr_stagger() via <sys/clockintr.h>.  They have external
  callers now.

- Delete pscnt, psdiv, psratio.  From schedstate_percpu, also delete
  spc_pscnt and spc_psdiv.  The statclock frequency is not dynamic
  anymore so these variables are now useless.

- Delete code/state related to the dynamic statclock frequency from
  kern_clockintr.c.  The statclock frequency can still be pseudo-random,
  so move the contents of clockintr_statvar_init() into clockintr_init().

With input from miod@, deraadt@, and claudio@.  Early revisions
cleaned up by claudio.  Early revisions tested by claudio@.  Tested by
cheloha@ on amd64, arm64, macppc, octeon, and sparc64 (sun4v).
Compile- and boot- tested on i386 by mlarkin@.  riscv64 compilation
bugs found by mlarkin@.  Tested on riscv64 by jca@.  Tested on
powerpc64 by gkoehler@.