[llvm] [CMake] Correctly handle LLVM_ENABLE_RUNTIMES in targets (PR #69869)

[Louis Dionne via llvm-commits]

ldionne wrote:

@petrhosek I kind of missed that change, but can you give an example of what this change is doing? What is an example of a "proxy" target?

https://github.com/llvm/llvm-project/pull/69869