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

[Joseph Huber via llvm-commits]

jhuber6 wrote:

I tried your suggested format after fixing the known places where CMake checks `if ("libc" IN_LISTS LLVM_ENABLE_RUNTIMES)` and it doesn't seem to work, I don't see the GPU triples in the runtime directory.

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