Symbolication of dynamic libraries doesn't work on MinGW

The test added in https://github.com/rust-lang/backtrace-rs/pull/332 is failing on MinGW. I believe the cause is [here](https://github.com/rust-lang/backtrace-rs/blob/1ca764ecabcd8db4e7a9c61ef73a7f22a6e779b8/src/symbolize/gimli.rs#L105-L120), which is that we don't actually attempt to load information about native libraries.