[llvm] [flang-rt] Disable host-side offload support when offload is disabled (PR #207998)

[Tom Eccles via llvm-commits]

tblah wrote:

> The `FLANG_RT_EXPERIMENTAL_OFFLOAD_SUPPORT` flag is not used _at all_ by our toolchain, this PR thus completely deletes the support for _all users_ except the CUDA offload one which _doesn't use it anyway._

So would it work for you if this used a new cmake flag with a different name which defaults to on?

https://github.com/llvm/llvm-project/pull/207998