kernel: Begin abstracting out _sched_spinlock

In a uniprocessor system, _sched_spinlock may not need to be
held in all the same cases that it does in a multiprocessor
system. Removing those unnecessary usages can lead to better
performance on UP systems. In the case of uncontested taking
and giving of a semaphore, this can be as much as a +14%
performance gain.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>