Summary:
Now that symbolizer has precise feature flags, enable
FOLLY_USE_SYMBOLIZER on any platform that has backtrace(3), which is
all that's needed to get a useful stack trace printed in the signal
handler.

Reviewed By: yfeldblum, luciang

Differential Revision: D23361609

fbshipit-source-id: b124f7fd01e94f90d8f8586555a64bd6e412040c