[llvm] [NFC] Check for defined(__GNUC__) before use (PR #116076)

[Markus Böck via llvm-commits]

https://github.com/zero9178 approved this pull request.

LGTM thank you! Ran into this recently as well and can confirm this fixes it

Might be worth being more precise in the commit message that this only happens when using `clang-cl` and `-Wundef`, which MLIR does eg. So this is even an in-tree issue

https://github.com/llvm/llvm-project/pull/116076