perf, tools, 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
Acked-by: Jiri Olsa <jolsa@kernel.org>
Signed-off-by: Andi Kleen <ak@linux.intel.com>