[llvm] Use zlib cmake target rather than variables (PR #90322)

[David Blaikie via llvm-commits]

dwblaikie wrote:

I don't know much about cmake, so hopefully some other folks weigh in (you might want to check on the change history of the files/lines you're touching, and see if you can find someone who might be a good reviewer that way)

But given this simplifies the code - that seems appealing.
But what's the cost, if any? Does this break some existing users/use cases?

https://github.com/llvm/llvm-project/pull/90322