perf stat: Use cpu-clock event for cpu targets

Currently 'perf stat' always counts task-clock event by default.  But
it's somewhat confusing for system-wide targets (especially with 'sleep
N' as the 'sleep' task just sleeps and doesn't use cputime).  Changing
to cpu-clock event instead for that case makes more sense IMHO.

Before:
  # perf stat -a sleep 0.1

   Performance counter stats for 'system wide':

        403.038603      task-clock (msec)     #    4.001 CPUs utilized
               150      context-switches      #    0.372 K/sec
                 7      cpu-migrations        #    0.017 K/sec
                71      page-faults           #    0.176 K/sec
        23,705,169      cycles                #    0.059 GHz
        15,888,166      instructions          #    0.67  insn per cycle
         3,326,078      branches              #    8.253 M/sec
            87,643      branch-misses         #    2.64% of all branches

       0.100737009 seconds time elapsed

  #

After:

  # perf stat -a sleep 0.1

   Performance counter stats for 'system wide':

        404.271182      cpu-clock (msec)      #    4.000 CPUs utilized
               143      context-switches      #    0.354 K/sec
                13      cpu-migrations        #    0.032 K/sec
                73      page-faults           #    0.181 K/sec
        22,119,220      cycles                #    0.055 GHz
        13,622,065      instructions          #    0.62  insn per cycle
         2,918,769      branches              #    7.220 M/sec
            85,033      branch-misses         #    2.91% of all branches

       0.101073089 seconds time elapsed

  #

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1463119263-5569-3-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>