Add support for using LLVM intrinsics instead of C atomics

[lorentey]

This adds a special build variant where C atomics are replaced with LLVM intrinsics.

This is not going to be enabled by default, because the use of -parse-stdlib is currently verboten for Swift packages.

However, it is an important step forward: once https://github.com/apple/swift/issues/56105 gets fixed, and we figure out how to allow the use of builtins from Swift packages, then this package could do away with defining a C module altogether.

Checklist

• [X] I've read the Contribution Guidelines
• [X] My contributions are licensed under the Swift license.
• [X] I've followed the coding style of the rest of the project.
• [ ] I've added tests covering all new code paths my change adds to the project (if appropriate).
• [X] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

Cc @glessard

[lorentey]

@swift-ci test