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

[Jon Chesterfield via llvm-commits]

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

I'm unconvinced by the runtime layer argument. Specifically, I'm interested in building libcxx for amdgpu without building libc for it, and it's plausible that libcxx on GPUs will not involve libunwind at all.

Likewise building compiler-rt for x64 and not nvptx, while building libc for nvptx and not x64 is probably a default configuration.

I would very much prefer cmake let us specify specific combinations instead of allowing a few blessed stacks.

GPU targets as cross compiling things, instead of as magic weird things with their own configuration, is the right direction.

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