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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

Any remaining concerns @petrhosek? I'd like to land this but would like to wait for your blessing.

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