perf pmus: Add placeholder core PMU

If loading a core PMU fails, legacy hardware/cache events may segv due
to there being no PMU. Create a placeholder empty PMU for this
case. This was discussed in:
https://lore.kernel.org/lkml/[email protected]/

Reported-by: Yang Jihong <[email protected]>
Tested-by: Yang Jihong <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Cc: Ravi Bangoria <[email protected]>
Cc: James Clark <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Suzuki Poulouse <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Ingo Molnar <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Namhyung Kim <[email protected]>

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