perf kmem: Print big numbers using thousands' group

Like perf stat, this makes easy to read the numbers on stat like below:

  # perf kmem stat

  SUMMARY
  =======
  Total bytes requested: 9,770,900
  Total bytes allocated: 9,782,712
  Total bytes wasted on internal fragmentation: 11,812
  Internal fragmentation: 0.120744%
  Cross CPU allocations: 74/152,819

Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Joonsoo Kim <js1304@gmail.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1427092244-22764-2-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>