MINOR: cpu-topo: ignore single-core clusters

MINOR: cpu-topo: ignore single-core clusters

Some platforms (several armv7, intel 14900 etc) report one distinct
cluster per core. This is problematic as it cannot let clusters be
used to distinguish real groups of cores, and cannot be used to build
thread groups.

Let's just compare the cluster cpus to the siblings, and ignore it if
they exactly match. We must also take care of not falling back to
core_cpus_list, which can enumerate cores that already have their
cluster assigned (e.g. intel 14900 has 4 4-Ecore clusters in addition
to the 8 Pcores).