[clang] [compiler-rt] [llvm] [CMake][compiler-rt] Support for using compiler-rt atomic library (PR #106603)

[Chris Apple via cfe-commits]

https://github.com/cjappl approved this pull request.

Seems reasonable to me, I would wait for other approvals, as I'm relatively new here.

Just ensure this is good to go with rtsan, I recommend running the tests locally, as I don't think they're a part of the default pre-commit set
```
ninja check-rtsan
```

https://github.com/llvm/llvm-project/pull/106603