perf stat: Move perf_counts struct and functions into separate object

perf stat: Move perf_counts struct and functions into separate object

Moving 'struct perf_counts' and associated functions into separate
object, so we could remove stat.c object dependency from python build.

It makes the python code to build properly, because it fails to load due
to missing stat-shadow.c object dependency if some patches from Kan
Liang are applied.

So apply this one, then Kan's.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20150807105103.GB8624@krava.brq.redhat.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>