[llvm] [LLVM] Add `LLVM_<proj>_RUNTIME_TARGETS` to set targets per-project (PR #81557)

[Joseph Huber via llvm-commits]

jhuber6 wrote:

So, I tired it out and it doesn't seem to have the semantics that I need. I want to be able to specify that only a specific project is built with the desired architecture, that means omitting it from the standard targets if present. That doesn't happen with the above `-DRUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES='libc'` method, because it  will then cause other runtimes like `openmp` or `libunwind` to be built for the GPU which is not desired. Additionally, that syntax is a lot more difficult to read and not as simple to put in a default build.

https://github.com/llvm/llvm-project/pull/81557