perf stat: Create the runtime_stat init/exit function

perf stat: Create the runtime_stat init/exit function

It mainly initializes and releases the rblist which is defined in struct
runtime_stat.

For the original rblist 'runtime_saved_values', it's still kept there
for keeping the patch bisectable.

The rblist 'runtime_saved_values' will be removed in later patch at
switching time.

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/1512482591-4646-4-git-send-email-yao.jin@linux.intel.com
[ Rename 'stat' variables to 'st' to build on centos:{5,6} and others where it shadows a global declaration ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>