perf tools: Introduce stat perf.data header feature

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 <[email protected]>
Tested-by: Kan Liang <[email protected]>
Cc: David Ahern <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>