perf stat: Save aggr value to first member of prev_raw_counts

perf stat: Save aggr value to first member of prev_raw_counts

To collect the overall statistics for interval mode, we copy the counts
from evsel->prev_raw_counts to evsel->counts.

For AGGR_GLOBAL mode, because the perf_stat_process_counter creates aggr
values from per cpu values, but the per cpu values are 0, so the
calculated aggr values will be always 0.

This patch uses a trick that saves the previous aggr value to the first
member of perf_counts, then aggr calculation in process_counter_values
can work correctly for AGGR_GLOBAL.

 v6:
 ---
 Add comments in perf_evlist__save_aggr_prev_raw_counts.

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/20200520042737.24160-5-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>