kernel - Add hw.cpu_mwait_haltand hw.cpu_mwait_spin

kernel - Add hw.cpu_mwait_haltand hw.cpu_mwait_spin

* Add globals with machine-specific meanings.  This allows us to operate
  with MONITOR/MWAIT and gain the same power savings that we get with
  ACPI halt mode when idle.  Waiting on a follow-up commit to set the
  the defaults.

* For example, if we sysctl hw.cpu_mwait_halt=0x44 hw.cpu_mwait_spin=0x11
  (and leaving machdep.cpu_idle_hlt=2) on a haswell cpu it will shave the
  idle power down by another 5W, from 25W to 20W on an idle Haswell Xeon
  3.4 GHz server chip without impacting wakeup performance too badly.