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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Also, like you said, this might not be explicitly useful beyond `libc` for now. I could simply make it a specific `LLVM_LIBC_RUNTIME_TARGETS` instead? But I'm planning ahead because I think in the future we will want a libc++ for the GPU that builds off of the libc for the GPU.

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