MINOR: cpu-topo: add a new "group-by-ccx" CPU policy

MINOR: cpu-topo: add a new "group-by-ccx" CPU policy

This cpu-policy will only consider CCX and not clusters. This makes
a difference on machines with heterogenous CPUs that generally share
the same L3 cache, where it's not desirable to create multiple groups
based on the CPU types, but instead create one with the different CPU
types. The variants "group-by-2/3/4-ccx" have also been added.

Let's also add some text explaining the difference between cluster
and CCX.