MINOR: cpu-topo: add new cpu-policies "group-by-2-clusters" and above

MINOR: cpu-topo: add new cpu-policies "group-by-2-clusters" and above

This adds "group-by-{2,3,4}-clusters", which, as its name implies,
create one thread group per X clusters. This can be useful when CPUs
are split into too small clusters, as well as when the total number
of assigned cores is not even between the clusters, to try to spread
the load between less different ones.