sched/cache: Make LLC id continuous

Introduce an index mapping between CPUs and their LLCs. This provides
a roughly continuous per LLC index needed for cache-aware load balancing in
later patches.

The existing per_cpu llc_id usually points to the first CPU of the
LLC domain, which is sparse and unsuitable as an array index. Using
llc_id directly would waste memory.

With the new mapping, CPUs in the same LLC share an approximate
continuous id:

  per_cpu(llc_id, CPU=0...15)  = 0
  per_cpu(llc_id, CPU=16...31) = 1
  per_cpu(llc_id, CPU=32...47) = 2
  ...

Note that the LLC IDs are allocated via bitmask, so the IDs may be
reused during CPU offline->online transitions.

Suggested-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Originally-by: K Prateek Nayak <kprateek.nayak@amd.com>
Co-developed-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Chen Yu <yu.c.chen@intel.com>
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/047ef46339e4db497b54a89940a7ebedf27fcf28.1775065312.git.tim.c.chen@linux.intel.com

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