[llvm] [CMake] Only export the LLVM_LINK_LLVM_DYLIB setting if not yet set (PR #135570)

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

Note that downstream users can chose by specifying `DISABLE_LLVM_LINK_LLVM_DYLIB` in their CMake

https://github.com/llvm/llvm-project/pull/135570