perf stat report: Allow to override aggr_mode

Allowing to override record aggr_mode. It's possible to use perf stat
like:

   $ perf stat report -A
   $ perf stat report --per-core
   $ perf stat report --per-socket

To customize the recorded aggregate mode regardless what was used during
the stat record command.

Reported-by: Kan Liang <kan.liang@intel.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1446734469-11352-19-git-send-email-jolsa@kernel.org


[ Renamed 'stat' parameter to 'st' to fix 'already defined' build error with older distros (e.g. RHEL6.7) ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>