sched_ext: cid: Represent clusters explicitly

The sched_ext CID topology gives each core, LLC and NUMA node a
contiguous CID range. Without cluster entries, schedulers cannot obtain
their CID range or global index.

Walk cores by cache-sharing cluster and add cluster_cid and cluster_idx
to scx_cid_topo. Treat the level inclusively: a core without a wider
cluster forms one of its own, while an LLC-wide cache-sharing group
forms one cluster. This gives every cluster a contiguous CID range and a
dense index.

The walk now takes cores in cluster order rather than LLC order. Shards
are still cut on core boundaries, so a shard doesn't need to contain a
whole cluster; only core, cluster, LLC and node are nested, in that
order.

The level follows whatever the arch reports through
topology_cluster_cpumask(), which on x86 is the L2 sharing mask and is
populated independently of CONFIG_SCHED_CLUSTER.

Tested on a 13th Gen Intel Core i7-13800H: six SMT P-cores occupy CPUs
0-11, eight E-cores occupy CPUs 12-19 in two four-core L2 modules, and
all 20 CPUs share one LLC; the P-core pairs form two-CID clusters, the
E-core modules form four-CID clusters, and cluster_idx is dense across
all eight clusters.

Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

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