BUG/MINOR: cpu-topo: fix group-by-cluster policy for disordered clusters

BUG/MINOR: cpu-topo: fix group-by-cluster policy for disordered clusters

Some (rare) boards have their clusters in an erratic order. This is
the case for the Radxa Orion O6 where one of the big cores appears as
CPU0 due to booting from it, then followed by the small cores, then the
medium cores, then the remaining big cores. This results in clusters
appearing this order: 0,2,1,0.

The core in cpu_policy_group_by_cluster() expected ordered clusters,
and performs ordered comparisons to decide whether a CPU's cluster has
already been taken care of. On the board above this doesn't work, only
clusters 0 and 2 appear and 1 is skipped.

Let's replace the cluster number comparison with a cpuset to record
which clusters have been taken care of. Now the groups properly appear
like this:

  Tgrp/Thr  Tid        CPU set
  1/1-2     1-2        2: 0,11
  2/1-4     3-6        4: 1-4
  3/1-6     7-12       6: 5-10

No backport is needed, this is purely 3.2.