[llvm] [CMake] Create generic runtimes targets as needed (PR #76096)

[Petr Hosek via llvm-commits]

petrhosek wrote:

I'm not sure if anyone uses these generic targets at all, it might be easier to just remove them altogether. I created #76097 which does that, let me know if you have a preference.

https://github.com/llvm/llvm-project/pull/76096