perf stat: Fix misleading message when specifying cpu list or system wide

perf stat: Fix misleading message when specifying cpu list or system wide

The "perf stat" tool displays the command run in its summary output
which is misleading when using a cpu list or system wide collection.

Before:

perf stat -a -- sleep 1

 Performance counter stats for 'sleep 1':

16152.670249 task-clock                #   16.132 CPUs utilized
         417 context-switches          #    0.002 M/sec
           7 cpu-migrations            #    0.030 K/sec
...

After:

perf stat -a -- sleep 1

 Performance counter stats for 'system wide':

16206.931120 task-clock                #   16.144 CPUs utilized
         395 context-switches          #    0.002 M/sec
           5 cpu-migrations            #    0.030 K/sec
...

or

perf stat -C1 -- sleep 1

 Performance counter stats for 'CPU(s) 1':

   1001.669257 task-clock                #    1.000 CPUs utilized
         4,264 context-switches          #    0.004 M/sec
             3 cpu-migrations            #    0.003 K/sec
...

Signed-off-by: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1380400080-9211-2-git-send-email-dsahern@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>