perf evsel: Restore evsel->priv as a tool private area

perf evsel: Restore evsel->priv as a tool private area

When we started using it for stats and did it not just in
builtin-stat.c, but also for builtin-script.c, then it stopped being a
tool private area, so introduce a new pointer for these stats and leave
->priv to its original purpose.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: yuzhoujian <yuzhoujian@didichuxing.com>
Fixes: cfc8874a4859 ("perf script: Process cpu/threads maps")
Link: http://lkml.kernel.org/n/tip-jtpzx3rjqo78snmmsdzwb2eb@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>