perf tools: Introduce stat perf.data header feature

Introducing the 'stat' feature to mark a perf.data as created by  the
'perf stat record' command. It contains no data.

It's needed so that the report tools (report/script) can differentiate
sampling data from counting data, because they need to be treated in a
different way.

In the future it might be used to store the version of the stat storage
system used.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Kan Liang <kan.liang@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1445784728-21732-28-git-send-email-jolsa@kernel.org


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