kernel - Refactor machdep.cpu_idle_hlt

kernel - Refactor machdep.cpu_idle_hlt

* Refactor machdep.cpu_idle_hlt.  Operation works essentially the
  same way as before but the code is a lot easier to read.

* Refactor mode 3 to have ACPI call back into machdep when entering
  a C1 halt.  Continue using HLT for this situation as we aren't
  really sure what ACPI may or may not have done.  MONITOR/MWAIT
  might not be appropriate.

* Fix bug in cpu_smp_stopped().  Just because cpu_mwait_hints is
  non-zero does not mean that there is an index entry for CPU_MWAIT_C1.
  Use the last entry based on cpu_mwait_hints instead.

* Note that machdep.cpu_idle_hlt mode 2 still doesn't work as intended.