Re-enable hardlinking using fclonefileat on macOS.

[stuhood]

On macOS (and on the same filesystem), Rust's std::fs::copy implementation uses fclonefileat, which is as performant as a hardlink, but more defensive (since the clones are completely independently of one another).

It also has the important benefit of playing nicely with Docker for macOS: so this change fixes #18162.

[jsirois]

I prefer the uses are inlined to their conditional compile location which means lifting a function - maybe inline. As is reads fine in a diff, but pretty haphazard just reading the file.