perf expr: Add has_event function

Some events are dependent on firmware/kernel enablement. Allow such
events to be detected when the metric is parsed so that the metric's
event parsing doesn't fail.

Signed-off-by: Ian Rogers <[email protected]>
Tested-by: Namhyung Kim <[email protected]>
Cc: Mark Rutland <[email protected]>
Cc: Eduard Zingerman <[email protected]>
Cc: Sohom Datta <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Caleb Biggers <[email protected]>
Cc: Edward Baker <[email protected]>
Cc: Perry Taylor <[email protected]>
Cc: Samantha Alt <[email protected]>
Cc: Weilin Wang <[email protected]>
Cc: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andrii Nakryiko <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Jing Zhang <[email protected]>
Cc: Kajol Jain <[email protected]>
Cc: Alexander Shishkin <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Zhengjun Xing <[email protected]>
Cc: John Garry <[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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