Begin introducing NUMA support for CPU threads and memory

by extending the core/platform API with some basic functionality.
The new functions allow:
 1. Determining how many NUMA nodes are available.
 2. Setting the executing thread to be bound to a particular node,
    or not bound at all.
 3. Allocating memory affiliated with a particular node.

This change introduces the API only, there is not yet a real
implementation.

PiperOrigin-RevId: 202724194