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

[Jon Chesterfield via llvm-commits]

JonChesterfield wrote:

Both `runtime=targets` and `target=runtimes` syntaxes look identically good/bad to me. I have to copy&paste cmake invocations from our documentation to get llvm to build anyway.

Keeping the current syntax and shipping the better behaviour today is much better than spending another week trying to make the syntax look different while the behavioural change bitrots.

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