Enable @examples//ffi/rust_calling_c/... on MacOS CI

Building these targets generates a build failure
```
rust_calling_c % bazel build ...
```
```
INFO: Invocation ID: f981af43-943e-40bb-ba01-ed9362446315
INFO: Analyzed 15 targets (0 packages loaded, 0 targets configured).
INFO: Found 15 targets...
ERROR: /private/var/tmp/_bazel_user/1426ad6c00f8f7b996ddc75bef70871c/external/local_libstdcpp/BUILD.bazel:3:11: SolibSymlink _solib_darwin_x86_64/_U@local_Ulibstdcpp_S_S_Clibstdc++___U/libstdc++.6.dylib failed: missing input file 'external/local_libstdcpp/libstdc++.6.dylib', owner: '@local_libstdcpp//:libstdc++.6.dylib'
ERROR: /private/var/tmp/_bazel_user/1426ad6c00f8f7b996ddc75bef70871c/external/local_libstdcpp/BUILD.bazel:3:11 SolibSymlink _solib_darwin_x86_64/_U@local_Ulibstdcpp_S_S_Clibstdc++___U/libstdc++.6.dylib failed: 1 input file(s) do not exist
INFO: Elapsed time: 0.415s, Critical Path: 0.01s
INFO: 11 processes: 11 internal.
FAILED: Build did NOT complete successfully
```

The examples should be updated (or potentially the underlying [rust_bindgen](https://bazelbuild.github.io/rules_rust/rust_bindgen.html) rules) to support MacOS.

Reproduce the failure with `bazel build ...` for `@examples//ffi/rust_calling_c/...` on macOS CI. Inspect the example targets and the underlying rust_bindgen rules referenced in the issue; done means these targets build successfully on macOS CI without the missing libstdc++.6.dylib error.

Written by the indexing model from the issue text.