Auto merge of #3856 - jder:mac-native-libs, r=RalfJung

Enable native libraries on macOS

Fixes #3595 by using `-fvisibility=hidden` and the visibility attribute supported by both gcc and clang rather than the previous gcc-only mechanism for symbol hiding. Also brings over cfg changes from #3594 which enable native-lib functionality on all unixes.

Thanks for taking a look, feedback very welcome!

cc `@RalfJung`

Author: bors