[clang] [llvm] Implement a more seamless way to provide missing functions on z/OS (PR #167703)

[LLVM Continuous Integration via llvm-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `libc-x86_64-debian-dbg-asan` running on `libc-x86_64-debian` while building `clang,llvm` at step 2 "update-annotated-scripts".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/147/builds/33275

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 2 (update-annotated-scripts) failure: update (failure)
git version 2.39.5
remote: Internal Server Error
fatal: unable to access 'https://github.com/llvm/llvm-zorg.git/': The requested URL returned error: 500
remote: Internal Server Error
fatal: unable to access 'https://github.com/llvm/llvm-zorg.git/': The requested URL returned error: 500

```

</details>

https://github.com/llvm/llvm-project/pull/167703