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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Also @petrhosek, I've noticed that the non-default targets don't use `PASSTHROUGH_PREFIXES`. Is there a reason for this? It's important to be able to pass arguments to the runtime. IMHO we should *always* pass `${canon_name}_` prefixes. I also have a need for `CUDA` so that users can pick up the CUDA Toolkit directory.

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