perf stat: Add per processor socket count aggregation

perf stat: Add per processor socket count aggregation

This patch adds per-processor socket count aggregation for system-wide
mode measurements. This is a useful mode to detect imbalance between
sockets.

To enable this mode, use --aggr-socket in addition
to -a. (system-wide).

The output includes the socket number and the number of online
processors on that socket. This is useful to gauge the amount of
aggregation.

 # ./perf stat -I 1000 -a --aggr-socket -e cycles sleep 2
 #           time socket cpus             counts events
      1.000097680 S0        4          5,788,785 cycles
      2.000379943 S0        4         27,361,546 cycles
      2.001167808 S0        4            818,275 cycles

Signed-off-by: Stephane Eranian <eranian@google.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1360161962-9675-3-git-send-email-eranian@google.com
[ committer note: Added missing man page entry based on above comments ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>