perf tools: Count evsel's event stat for multi thread

Add ->multi_stats field to struct hists to avoid a race between
multple threads.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
perf tools: Defer dso loaded flag set

Since other thread can see it before actually loading symbol from a
dso so it needs to be set at last.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>