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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

> You can already do the following:
> 
> ```
> -DLLVM_RUNTIME_TARGETS='nvptx64-nvidia-cuda;amdgcn-amd-amdhsa'
> -DRUNTIMES_nvptx64-nvidia-cuda_LLVM_ENABLE_RUNTIMES='libc'
> -DRUNTIMES_amdgcn-amd-amdhsa_LLVM_ENABLE_RUNTIMES='libc'
> ```
> 
> This is already [used in existing builds](https://github.com/llvm/llvm-project/blob/bc66e0cf9feb8f3c38fa2e33850b6c6e5477fb21/clang/cmake/caches/Fuchsia-stage2.cmake#L333). Is that sufficient for your use case?

Will that allow the other targets to still be built? This approach seems more straightforward honestly, but if it's sufficient I could make it work.

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