perf tools: Add front cache for dso data access

There's a high contention in dso_cache__find() due to the dso->lock
when dwarf unwinding is done with libunwind.  Add last accessed
pointers of dso_cache to lockless lookup.  It'll fallback to normal
tree search when it misses the last cache.

The size 16 is arbitrary and works best for my setting.

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