perf list: Avoid a hardcoded cpu PMU name

perf list: Avoid a hardcoded cpu PMU name

Use the first core PMU instead.

On a Raspberry Pi, before:

  $ perf list
  ...
    cpu/t1=v1[,t2=v2,t3 ...]/modifier                  [Raw hardware event descriptor]
         [(see 'man perf-list' on how to encode it)]
  ...

After:

  $ perf list
  ...
    armv8_cortex_a72/t1=v1[,t2=v2,t3 ...]/modifier     [Raw hardware event descriptor]
         [(see 'man perf-list' on how to encode it)]
  ...
  ```

Signed-off-by: Ian Rogers <irogers@google.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kang Minchul <tegongkang@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Richter <tmricht@linux.ibm.com>
Cc: Xing Zhengjun <zhengjun.xing@linux.intel.com>
Link: https://lore.kernel.org/r/20230906234416.3472339-1-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>