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

[Jonas Rembser via llvm-commits]

guitargeek wrote:

Correct, that could be a possible workaround on the ROOT side. But whatever is done in ROOT, that doesn't change the fact that the overwriting mechanism in `llvm/cmake/modules/CMakeLists.txt` -- which according to the [commit message](https://github.com/llvm/llvm-project/commit/5fed24a39698466952504850e8fd7703e17ce679) is there to get the "correct default", -- is not very helpful.

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