perf pmus: Add notion of default PMU for JSON events

perf pmus: Add notion of default PMU for JSON events

JSON events created in pmu-events.c by jevents.py may not specify a
PMU they are associated with, in which case it is implied that it is
the first core PMU. Care is needed to select this for regular 'cpu',
s390 'cpum_cf' and ARMs many names as at the point the name is first
needed the core PMUs list hasn't been initialized. Add a helper in
perf_pmus to create this value, in the worst case by scanning sysfs.

v2. Add missing close if fdopendir fails.

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Thomas Richter <tmricht@linux.ibm.com>
Cc: Ravi Bangoria <ravi.bangoria@amd.com>
Cc: James Clark <james.clark@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Link: https://lore.kernel.org/r/20230623043843.4080180-1-irogers@google.com
Signed-off-by: Namhyung Kim <namhyung@kernel.org>