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

[Louis Dionne via llvm-commits]

ldionne wrote:

I don't remember that breakage, unfortunately. We shouldn't be using the bootstrapping build internally, so I don't *think* this should impact us, and I don't have any objection against this change.

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