perf stat: Extend rbtree to support per-thread shadow stats

perf stat: Extend rbtree to support per-thread shadow stats

Previously the rbtree was used to link generic metrics.

This patches adds new ctx/type/stat into rbtree keys because we will use
this rbtree to maintain shadow metrics to replace original a couple of
static arrays for supporting per-thread shadow stats.

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/1512482591-4646-3-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>