perf bench: Add a multi-threaded synthesize benchmark

By default this isn't run as it reads /proc and may not have access.
For consistency, modify the single threaded benchmark to compute an
average time per event.

Committer testing:

  $ grep -m1 "model name" /proc/cpuinfo
  model name	: Intel(R) Core(TM) i7-8650U CPU @ 1.90GHz
  $ grep "model name" /proc/cpuinfo  | wc -l
  8
  $
  $ perf bench internals synthesize -h
  # Running 'internals/synthesize' benchmark:

   Usage: perf bench internals synthesize <options>

      -I, --multi-iterations <n>
                            Number of iterations used to compute multi-threaded average
      -i, --single-iterations <n>
                            Number of iterations used to compute single-threaded average
      -M, --max-threads <n>
                            Maximum number of threads in multithreaded bench
      -m, --min-threads <n>
                            Minimum number of threads in multithreaded bench
      -s, --st              Run single threaded benchmark
      -t, --mt              Run multi-threaded benchmark

  $
  $ perf bench internals synthesize -t
  # Running 'internals/synthesize' benchmark:
  Computing performance of multi threaded perf event synthesis by
  synthesizing events on CPU 0:
    Number of synthesis threads: 1
      Average synthesis took: 65449.000 usec (+- 586.442 usec)
      Average num. events: 9405.400 (+- 0.306)
      Average time per event 6.959 usec
    Number of synthesis threads: 2
      Average synthesis took: 37838.300 usec (+- 130.259 usec)
      Average num. events: 9501.800 (+- 20.469)
      Average time per event 3.982 usec
    Number of synthesis threads: 3
      Average synthesis took: 48551.400 usec (+- 225.686 usec)
      Average num. events: 9544.000 (+- 0.000)
      Average time per event 5.087 usec
    Number of synthesis threads: 4
      Average synthesis took: 29632.500 usec (+- 50.808 usec)
      Average num. events: 9544.000 (+- 0.000)
      Average time per event 3.105 usec
    Number of synthesis threads: 5
      Average synthesis took: 33920.400 usec (+- 284.509 usec)
      Average num. events: 9544.000 (+- 0.000)
      Average time per event 3.554 usec
    Number of synthesis threads: 6
      Average synthesis took: 27604.100 usec (+- 72.344 usec)
      Average num. events: 9548.000 (+- 0.000)
      Average time per event 2.891 usec
    Number of synthesis threads: 7
      Average synthesis took: 25406.300 usec (+- 933.371 usec)
      Average num. events: 9545.500 (+- 0.167)
      Average time per event 2.662 usec
    Number of synthesis threads: 8
      Average synthesis took: 24110.400 usec (+- 73.229 usec)
      Average num. events: 9551.000 (+- 0.000)
      Average time per event 2.524 usec
  $

Signed-off-by: Ian Rogers <irogers@google.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@redhat.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andrey Zhizhikin <andrey.z@gmail.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Kefeng Wang <wangkefeng.wang@huawei.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Stephane Eranian <eranian@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lore.kernel.org/lkml/20200415054050.31645-2-irogers@google.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>