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

[Petr Hosek via llvm-commits]

petrhosek wrote:

AFAICT pull/81921 is not dependent on this change, this change just adds syntax sugar on top of the existing functionality provided by the runtimes build and so shouldn't be considered a blocker.

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