[llvm] [Offload] Only issue a warning for building `offload` on the GPU and return (PR #179240)

[Michael Kruse via llvm-commits]

Meinersbur wrote:

If it does not work, shouldn't this be an error? A warning might disappear between other CMake messages.

Why is the title "Only issue a warning", it doesn't seem remove any build errors?

https://github.com/llvm/llvm-project/pull/179240