[libclc] [llvm] [libclc] Support LLVM_ENABLE_RUNTIMES when building (PR #141574)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

The changes make sense for just adding it, but does it actually work? The CMake I've seen in `libclc` does multiple compilations and some with `--target=amdgcn` for example, which is a little different from how the runtimes builds handle it.

Do you have an example of how to actually build this stuff?

https://github.com/llvm/llvm-project/pull/141574