Add SMP Priority Scheduler with Affinity

This scheduler attempts to account for needed thread migrations caused
as a side-effect of a thread state, affinity, or priority change operation.

This scheduler has its own allocate_processor handler named
_Scheduler_SMP_Allocate_processor_exact() because
_Scheduler_SMP_Allocate_processor() attempts to prevent an executing
thread from moving off its current CPU without considering affinity.
Without this, the scheduler makes all the right decisions and then
they are discarded at the end.

==Side Effects of Adding This Scheduler==

Added Thread_Control * parameter to Scheduler_SMP_Get_highest_ready type
so methods looking for the highest ready thread can filter by the processor
on which the thread blocking resides. This allows affinity to be considered.
Simple Priority SMP and Priority SMP ignore this parameter.

+ Added get_lowest_scheduled argument to _Scheduler_SMP_Enqueue_ordered().

+ Added allocate_processor argument to the following methods:
  - _Scheduler_SMP_Block()
  - _Scheduler_SMP_Enqueue_scheduled_ordered()
  - _Scheduler_SMP_Enqueue_scheduled_ordered()

+ schedulerprioritysmpimpl.h is a new file with prototypes for methods
  which were formerly static in schedulerprioritysmp.c but now need to
  be public to be shared with this scheduler.

NOTE:
  _Scheduler_SMP_Get_lowest_ready() appears to have a path which would
  allow it to return a NULL.  Previously, _Scheduler_SMP_Enqueue_ordered()
  would have asserted on it. If it cannot return a NULL,
  _Scheduler_SMP_Get_lowest_ready() should have an assertions.