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

[Petr Hosek via llvm-commits]

petrhosek 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#L136). Is that sufficient for your use case?

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