kernel - Add MONITOR/MWAIT support to the LWKT scheduler

kernel - Add MONITOR/MWAIT support to the LWKT scheduler

* Adjust the FIFO contention resequencer (which deals with spinning
  on tokens) to use MONITOR/MWAIT when available instead of DELAY(1)
  when waiting to become the head of the queue.

* Adjust the x86-64 idle loop to use MONITOR/MWAIT when available when
  the idle halt mode (machdep.cpu_idle_hlt) is set to 1.  This
  significantly improves performance for event-oriented programs, including
  compile pipelines.

  NOTE: On the 48-core monster setting machdep.cpu_idle_hlt to 1 improves
  performance but at the cost of an additional 200W of power at idle vs
  the default value of 2 (ACPI idle halt).  Look for a hybrid approach in
  a future commit.