folly: symbolizer: increase default signal-handler symbolizer elf file cache size...

folly: symbolizer: increase default signal-handler symbolizer elf file cache size (make it work with large number of .so)

Summary:
Bump the default size so we can symbolize symbols in dynamically linked
binaries with lots and lots of shared libraries :)

folly/experimental/symbolizer/ElfCache.h
```
  * Will not grow; once the capacity is reached, lookups for files that
  * aren't already in the cache will fail (return nullptr).
  class SignalSafeElfCache
```

Reviewed By: ot, philippv

Differential Revision: D3984649

fbshipit-source-id: acdca9b9adf6c16f3a1556bc3f57b28844cdd9cc