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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

This is a prereq for pull/81921 which is a huge decrease in overall complexity of our build scripts. Hopefully @petrhosek has some more ideas for cleaning up this area that we can apply post-commit.

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