configure: Don't attempt to override linker on Darwin

Configure's --enable-ld-override functionality is intended to ensure that we don't rely on ld.bfd, which tends to be slow and buggy, on Linux and Windows. However, on Darwin the lack of sensible package management makes it extremely easy for users to have awkward mixtures of toolchain components from, e.g., XCode, the Apple Command-Line Tools package, and homebrew. This leads to extremely confusing problems like #21712 (closed).

Here we avoid this by simply giving up on linker selection on Darwin altogether. This isn't so bad since the Apple ld64 linker has decent performance and AFAICT fairly reliable.

Closes #21712 (closed).