WiP: cpuallocator: use last useful cache for clustering.

Try finding the last cache which provides non-trivial clustering
of CPUs and use that for cache-group based allocation.

Note that this current implementation is somewhat simplistic. It
expects all CPUs to provide identical cache grouping and picks a
single cache level to set up clusters. This might result in sub-
optimal cache-based clustering on hybrid core architectures. We
can address this shortcoming in the future.

Signed-off-by: Krisztian Litkey <[email protected]>