enable FOLLY_USE_SYMBOLIZER on all non-Windows builds
enable FOLLY_USE_SYMBOLIZER on all non-Windows builds

Summary: The only thing FOLLY_USE_SYMBOLIZER guards now is whether to install signal handlers, and we can do that on any unix.

Reviewed By: yfeldblum, luciang

Differential Revision: D23881199

fbshipit-source-id: 45631b59fe5df9656a227b68683f23d4ff65e223

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly