kernel/sched: Add CONFIG_CPU_MASK_PIN_ONLY

Some SMP applications have threading designs where every thread
created is always assigned to a specific CPU, and never want to
schedule them symmetrically across CPUs under any circumstance.

In this situation, it's possible to optimize the run queue design a
bit to put a separate queue in each CPU struct instead of having a
single global one.  This is probably good for a few cycles per
scheduling event (maybe a bit more on architectures where cache
locality can be exploited) in circumstances where there is more than
one runnable thread.  It's a mild optimization, but a basically simple
one.

Signed-off-by: Andy Ross <andrew.j.ross@intel.com>