Updated Bazelmod cross compilation to catch MacOS sys dependency issues

[marvin-hansen]

As discussed in issue #2917,
this updated cross compilation example serves as a bridge until the exact cause of the sys crates not building on Mac issue has been found and better regression testing has been added.

Also, I took the freedom to trim and streamline the LLVM setup making it easier to configure while preserving the property that it caches failing sys crate builds. Updated the Readme accordingly.

[marvin-hansen]

Update:

• Apparently none of the Bazelmod examples run on MacOS on CI; at least that explains how things slipped through.
• I added one MacOS test runner for the cross compile example following the format of all the other tests.
• Since the MacOS runner uses X86, I updated the LLVM config so that Bazel downloads the correct Intel/Mac version.

However, I am not familiar with BazelCI, but is there a way to add a single MacOS runner that basically runs all Bazelmod tests similar to the crate universe runner for the Workspace examples?

Would be great if someone with a solid understanding of the CI would ensure that all Bazelmod examples get build and tested on MacOS.