perf stat: Check existence of frontend/backed stalled cycles

Only put the frontend/backend stalled cycles into the default perf stat
events when the CPU actually supports them.

This avoids empty columns with --metric-only on newer Intel CPUs.

Committer note:

Before:

  $ perf stat ls

    Performance counter stats for 'ls':

          1.080893     task-clock (msec)      #    0.619 CPUs utilized
                 0     context-switches       #    0.000 K/sec
                 0     cpu-migrations         #    0.000 K/sec
                97     page-faults            #    0.090 M/sec
         3,327,741     cycles                 #    3.079 GHz
   <not supported>     stalled-cycles-frontend
   <not supported>     stalled-cycles-backend
         1,609,544     instructions           #    0.48  insn per cycle
           319,117     branches               #  295.235 M/sec
            12,246     branch-misses          #    3.84% of all branches

       0.001746508 seconds time elapsed
  $

After:

  $ perf stat ls

    Performance counter stats for 'ls':

          0.693948     task-clock (msec)      #    0.662 CPUs utilized
                 0     context-switches       #    0.000 K/sec
                 0     cpu-migrations         #    0.000 K/sec
                95     page-faults            #    0.137 M/sec
         1,792,509     cycles                 #    2.583 GHz
         1,599,047     instructions           #    0.89  insn per cycle
           316,328     branches               #  455.838 M/sec
            12,453     branch-misses          #    3.94% of all branches

       0.001048987 seconds time elapsed
  $

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/1456532881-26621-2-git-send-email-andi@firstfloor.org


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