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

[Shoaib Meenai via llvm-commits]

https://github.com/smeenai approved this pull request.

Ah, I forgot about the opposite case, where you're only adding a runtime for a specific target. This LGTM then.

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