perf report: Make --skip-empty as default

perf report: Make --skip-empty as default

so that the compact output is shown by default.  Also add 'report.skip-empty'
config option to override the default.  Users can also use --no-skip-empty
command line option to change the behavior anytime.

Committer testing:

  $ perf report --stat

  Aggregated stats:
             TOTAL events:         19
              COMM events:          2
              EXIT events:          1
            SAMPLE events:          8
             MMAP2 events:          4
    FINISHED_ROUND events:          1
        THREAD_MAP events:          1
           CPU_MAP events:          1
         TIME_CONV events:          1
  cycles:u stats:
            SAMPLE events:          8
  $ perf config report.skip-empty=false
  $ perf report --stat

  Aggregated stats:
             TOTAL events:         19
              MMAP events:          0
              LOST events:          0
              COMM events:          2
              EXIT events:          1
          THROTTLE events:          0
        UNTHROTTLE events:          0
              FORK events:          0
              READ events:          0
            SAMPLE events:          8
             MMAP2 events:          4
               AUX events:          0
      ITRACE_START events:          0
      LOST_SAMPLES events:          0
            SWITCH events:          0
   SWITCH_CPU_WIDE events:          0
        NAMESPACES events:          0
           KSYMBOL events:          0
         BPF_EVENT events:          0
            CGROUP events:          0
         TEXT_POKE events:          0
              ATTR events:          0
        EVENT_TYPE events:          0
      TRACING_DATA events:          0
          BUILD_ID events:          0
    FINISHED_ROUND events:          1
          ID_INDEX events:          0
     AUXTRACE_INFO events:          0
          AUXTRACE events:          0
    AUXTRACE_ERROR events:          0
        THREAD_MAP events:          1
           CPU_MAP events:          1
       STAT_CONFIG events:          0
              STAT events:          0
        STAT_ROUND events:          0
      EVENT_UPDATE events:          0
         TIME_CONV events:          1
           FEATURE events:          0
        COMPRESSED events:          0
  cycles:u stats:
            SAMPLE events:          8
  $ perf config report.skip-empty
  report.skip-empty=false
  $

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/20210427013717.1651674-6-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>