perf metrics: Add has_pmem literal

Add literal so that if nvdimms aren't installed we can record fewer
events.  The file detection mechanism was suggested by Dan Williams
<[email protected]> in:

  https://lore.kernel.org/linux-perf-users/[email protected]/

Reviewed-by: Kan Liang <[email protected]>
Signed-off-by: Ian Rogers <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Caleb Biggers <[email protected]>
Cc: Dan Williams <[email protected]>
Cc: Edward Baker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Perry Taylor <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Samantha Alt <[email protected]>
Cc: Weilin Wang <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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