[_AtomicShims] On Darwin, call swift_retain_n/swift_release_n via dlsym

[lorentey]

This avoids a linker failure when the build system decides to build this module as a standalone library.

Hopefully this will all go away soon, when swift_retain_n/swift_release_n will become primitives exposed by the stdlib.

rdar://108390931

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

[lorentey]

Quick search indicates that it's okay, at least as long as dlopen/dlsym is only used to load specific, hardwired symbols from public libraries. So it appears this will be fine.

[lorentey]

Closing in favor of #97.