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

[Shoaib Meenai via llvm-commits]

smeenai wrote:

Apologies for the radio silence here, I'm about to go on an extended leave soon and have had lots to wrap up before then. I generally agree with Petr that adding another configuration option (and in particular one that goes from project -> target, rather than target -> project) is adding extra testing burden that we'd ideally find a way to avoid. I haven't been able to look at the other diffs to weigh that against the simplifications permitted by this though.

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