MINOR: cpu-topo: provide a function to sort clusters by average capacity

MINOR: cpu-topo: provide a function to sort clusters by average capacity

The current per-capacity sorting function acts on a whole cluster, but
in some setups having many small cores and few big ones, it becomes
easy to observe an inversion of metrics where the many small cores show
a globally higher total capacity than the few big ones. This does not
necessarily fit all use cases. Let's add new a function to sort clusters
by their per-cpu average capacity to cover more use cases.