perf stat: Copy counts from prev_raw_counts to evsel->counts

perf stat: Copy counts from prev_raw_counts to evsel->counts

It would be useful to support the overall statistics for perf-stat
interval mode. For example, report the summary at the end of "perf-stat
-I" output.

But since perf-stat can support many aggregation modes, such as
--per-thread, --per-socket, -M and etc, we need a solution which doesn't
bring much complexity.

The idea is to use 'evsel->prev_raw_counts' which is updated in each
interval and it's saved with the latest counts. Before reporting the
summary, we copy the counts from evsel->prev_raw_counts to
evsel->counts, and next we just follow non-interval processing.

 v5:
 ---
 Don't save the previous aggr value to the member of [cpu0,thread0]
 in perf_counts. Originally that was a trick because the
 perf_stat_process_counter would create aggr values from per cpu
 values. But we don't need to do that all the time. We will
 handle it in next patch.

Signed-off-by: Jin Yao <yao.jin@linux.intel.com>
Reviewed-by: Jiri Olsa <jolsa@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Jin Yao <yao.jin@intel.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lore.kernel.org/lkml/20200520042737.24160-4-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>