[llvm] llvm-shlib: Fix libLLVM-${MAJOR}.so symlink on MacOS (PR #85163)

[Martin Storsjö via llvm-commits]

mstorsjo wrote:

> @mstorsjo What symlinks / library names do you expect for mingw?

I see the other recent changes here have regressed how libLLVM is set up for mingw, #82660 and/or #79376), so I see this behaves worse for 18.1.0 final, while it was ok in earlier RCs.

The ideal situation for mingw is like it was before all these changes; `bin/libLLVM-18.dll` coupled with `lib/libLLVM.dll.a`, no symlinks needed.

https://github.com/llvm/llvm-project/pull/85163