[llvm] [llvm] Enable LLVM_LINK_LLVM_DYLIB by default on non-Windows platforms (PR #138187)

[Saleem Abdulrasool via llvm-commits]

https://github.com/compnerd approved this pull request.

I'm not a big fan of the naming as `LLVM_LINK_LLVM_DYLIB` and `LLVM_BUILD_LLVM_DYLIB` are easy to mix up in a quick read. However, it also makes sense to name them similarly for the configuration behaviour. I'm not sure if there is a great solution here, but pointing this out just incase someone happens to have some brilliant inishgt.

https://github.com/llvm/llvm-project/pull/138187