perf stat: Check for frontend stalled for metrics

Add an extra check for frontend stalled in the metrics.  This avoids an
extra column for the --metric-only case when the CPU does not support
frontend stalled.

v2: Add separate init function

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Link: http://lkml.kernel.org/r/1456858672-21594-8-git-send-email-andi@firstfloor.org


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