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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Would it be possible to make this move along faster if I just implemented it as `LLVM_LIBC_RUNTIME_TARGETS` and handled it specially there? I have the GPU `libc` rewrite done so I'm hoping to land it and get to work updating everything else since that patch solves pretty much every remaining issue I had before I could say the GPU `libc` was "done".

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