perf stat: Allocate shadow stats buffer for threads

perf stat: Allocate shadow stats buffer for threads

After perf_evlist__create_maps() being executed, we can get all threads
from /proc. And via thread_map__nr(), we can also get the number of
threads.

With the number of threads, the patch allocates a buffer which will
record the shadow stats for these threads.

The buffer pointer is saved in stat_config.

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/1512482591-4646-8-git-send-email-yao.jin@linux.intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>