perf report: Print percentage of each event statistics

It's sometimes useful to see how many samples vs other events in the
data file with percent values.

  $ perf report --stat

  Aggregated stats:
             TOTAL events:      20064
              MMAP events:        239  ( 1.2%)
              COMM events:       1518  ( 7.6%)
              EXIT events:          1  ( 0.0%)
              FORK events:       1517  ( 7.6%)
            SAMPLE events:       4015  (20.0%)
             MMAP2 events:      12769  (63.6%)
    FINISHED_ROUND events:          2  ( 0.0%)
        THREAD_MAP events:          1  ( 0.0%)
           CPU_MAP events:          1  ( 0.0%)
         TIME_CONV events:          1  ( 0.0%)
  cycles stats:
            SAMPLE events:       2475
  instructions stats:
            SAMPLE events:       1540

Suggested-by: Andi Kleen <ak@linux.intel.com>
Reviewed-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20210427013717.1651674-7-namhyung@kernel.org


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