perf jevents: Use "default_core" for events with no Unit

The JSON Unit field encodes the name of the PMU to match the events
to. When no name is given it has meant the "cpu" core PMU except for
tests.

On ARM, Intel hybrid and s390 the core PMU is named differently which
means that using "cpu" for this case causes the events not to get
matched to the PMU.

Introduce a new "default_core" string for this case and in the
pmu__name_match force all core PMUs to match this name.

Fixes: 2e255b4f

 ("perf jevents: Group events by PMU")
Reported-by: Arnaldo Carvalho de Melo <acme@kernel.org>
Reported-by: Thomas Richter <tmricht@linux.ibm.com>
Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jing Zhang <renyu.zj@linux.alibaba.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: John Garry <john.g.garry@oracle.com>
Cc: Kajol Jain <kjain@linux.ibm.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20230826062203.1058041-1-irogers@google.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>