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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

@petrhosek please shout out if you want to block this in the current form - I have no doubt that the cmake could be better, but that's always true, and this would really help the gpu project. When someone sees a better way to express the same capability we'll be delighted to adopt it.

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