cc_library dependencies aren't loaded on MacOS

GHC expects shared libraries to be `.dylib` on MacOS, but Bazel cc_library rules always emit `.so`:

```
$ hrepl hrepl/tests:CLib1
...
GHCi, version 8.6.5: http://www.haskell.org/ghc/ :? for help
: user specified .o/.so/.DLL could not be loaded (dlopen(libhrepl_Stests_Slibclib1.dylib, 5): image not found)
Whilst trying to load: (dynamic) hrepl_Stests_Slibclib1
Additional directories searched: bazel-out/darwin-fastbuild/bin/_solib_darwin
Error running GHCi.
```

`rules_haskell` uses `cc_wrapper` to work around this. We can probably do something similar for hrepl.