MINOR: cpu-topo: implement a sorting mechanism by CPU locality

MINOR: cpu-topo: implement a sorting mechanism by CPU locality

Once we've kept only the CPUs we want, the next step will be to form
groups and these ones are based on locality. Thus we'll have to sort by
locality. For now the locality is only inferred by the index. No grouping
is made at this point. For this we add the "cpu_reorder_by_locality"
function with a locality-based comparison function.