[CMake] Do not list .gyb files as sources

[lorentey]

Sources/Atomics/CMakeLists.txt currently lists .gyb files in its source lists, as well as the swift sources expanded from them. This doesn't seem to prevent the build from succeeding, but it seems undesirable.

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]

@swift-ci test