Make AtomicsShims compatible with C++ interop

[egorzhdan]

This fixes linker errors such as

Undefined symbols for architecture arm64:
  "_sa_retain_n(void*, unsigned int)", referenced from:
      (extension in Atomics):Swift.Unmanaged.retain(by: Swift.Int) -> () in Unmanaged extensions.swift.o
   NOTE: found '__sa_retain_n' in _AtomicsShims.c.o, declaration possibly missing 'extern "C"'

when building with Swift/C++ interoperability enabled.

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.