[_AtomicShims] On Darwin, call swift_retain_n/swift_release_n via inline assembly

[lorentey]

This is an alternative to #95, using another, far more fascinating, way to attempt to resolve #55. It uses the C header smuggle a call to swift_retain_n/swift_release_n behind the back of the Swift ARC optimizer using inline assembly.

(This draft generates procedure calls; it would probably be better to jump directly into the runtime.)

Resolves #55.

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).
• [ ] I've verified that my change does not break any existing tests.
• [ ] I've updated the documentation if necessary.

[lorentey]

@swift-ci test

[lorentey]

Closing in favor of #97.