lib/os: Add sys_heap_usable_size()

Add a simple internal block size predicate to expose the internal
memory region reserved for an allocation.  The immediate use case is
cache-incoherent systems wanting to do an invalidate of freed memory,
but it might be useful for apps doing e.g. string processing to better
optimize size changes, etc...

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