Gimli doesn't support fat libraries on macOS

[alexcrichton]

I noticed in #317 that the tests fail on macOS with the Gimli implementation, meaning that it's currently relying on the dladdr fallback, which is bad!

I tracked this down to a lack of support for fat libraries because the system libraries were failing to parse as a mach-o image (because they weren't!). I've implemented this in 69db3b7 but wanted to track this in any case.