Add a weakCompareExchange variant that only takes a single ordering

[lorentey]

Resolves #75

Checklist

• I've read the Contribution Guidelines
• My contributions are licensed under the Swift license.
• 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).
• I've verified that my change does not break any existing tests.
• I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[glessard]

I agree these are useful to have. 🚢