perf stat: Update or print per-thread stats

If the stats pointer in stat_config structure is not null, it will
update the per-thread stats or print the per-thread stats on this
buffer.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1512482591-4646-9-git-send-email-yao.jin@linux.intel.com


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