lib/os: P4 Work Queue: Pooled Parallel Preemptible Priority-based

This adds a somewhat special purpose IPC mechanism.  It's intended for
applications which have a "work queue" like architecture of discrete
callback items, but which need the ability to schedule those items
independently in separate threads across multiple CPUs.  So P4 Work
items:

1. Can run at any Zephyr scheduler priority and with any deadline
   (this feature assumes EDF scheduling is enabled)

2. Can be submitted at any time and from any context, including being
   resubmitted from within their own handler.

3. Will preempt any lower priority work as soon as they are runnable,
   according to the standard rules of Zephyr priority scheduling.

4. Run from a pool of worker threads that can be allocated efficiently
   (i.e. you need as many as the number of CPUs plus the number of
   preempted in-progress items, but no more).

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