[llvm] [offload] `gnu::format` with variadic template functions is Clang-only (PR #124406)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 approved this pull request.

I suppose most never noticed because the runtime build uses clang. That being said I don't think we should prevent the offloading runtime from being built with other compilers. Thanks.

https://github.com/llvm/llvm-project/pull/124406