perf hists: Add support for accumulated stat of hist entry

perf hists: Add support for accumulated stat of hist entry

Maintain accumulated stat information in hist_entry->stat_acc if
symbol_conf.cumulate_callchain is set.  Fields in ->stat_acc have same
vaules initially, and will be updated as callchain is processed later.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arun Sharma <asharma@fb.com>
Tested-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Link: http://lkml.kernel.org/r/1401335910-16832-4-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>