kernel/sched: CPU mask affinity/pinning API

This adds a simple implementation of SMP CPU affinity to Zephyr.  The
API is simple and doesn't try to invent abstractions like "cpu sets".
Each thread has an enable/disable flag associated with each CPU in the
system, and the bits can be turned on and off (for threads that are
not currently runnable, of course) using an easy three-function API.

Because the implementation picked requires enumerating runnable
threads in priority order looking for one that match the current CPU,
this is not a good fit for the SCALABLE or MULTIQ scheduler backends,
so it currently can be enabled only for SCHED_DUMB (which is the
default anyway).  Fancier algorithms do exist, but even the best of
them scale as O(N_CPUS), so aren't quite constant time and often
require significant memory overhead to keep separate lists for
different cpus/sets.

The intended use here is for apps that want to "pin" threads to
specific CPUs for latency control, or conversely to prevent certain
threads from taking time on specific CPUs to leave them free for fast
response.

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