lib/os: Add sys_multi_heap utility

This is a simple wrapper allowing multiple sys_heap regions to be
unified under a single allocation API.  Sometimes apps need the
ability to share multiple discontiguous regions in a single "heap", or
to have memory of different "types" be allocated heuristically based
on usage (e.g. cacheability, latency, power...).  This allows a
user-specified function to select the underlying memory to use for
each application.

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