perf tools: Add stat config event synthesize function

Introduce the perf_event__synthesize_stat_config to synthesize a 'struct
perf_stat_config'.

Storing the stat config in the form of tag-value pairs will, I believe,
sort out future version extensibility issues.

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/1445784728-21732-13-git-send-email-jolsa@kernel.org


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