perf list: Don't print Unit for "default_core"

perf list: Don't print Unit for "default_core"

"default_core" was added as a way to demark JSON events whose PMU should
be whatever the default core PMU is, previously this had been assumed to
be "cpu" but that fails on s390 and ARM.

'perf list' displays the PMU in the event description to save storing it
in JSON, but was still comparing against "cpu" and not "default_core",
so update this.

Fixes: d2045f87154bf67a ("perf jevents: Use "default_core" for events with no Unit")
Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Kan Liang <kan.liang@linux.intel.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: James Clark <james.clark@arm.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20230831071421.2201358-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>