perf stat: Define a structure for per-thread shadow stats

Perf has a set of static variables to record the runtime shadow metrics
stats.

While if we want to record the runtime shadow stats for per-thread, it
will be the limitation. This patch creates a structure and the next
patches will use this structure to update the runtime shadow stats for
per-thread.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1512482591-4646-2-git-send-email-yao.jin@linux.intel.com


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