Investigate preprocessor issues when parsing assembly

[hasufell]

When building libffi-clib, we get preprocessor errors on multiple platforms.

Upstream ticket: libffi/libffi#950

Supposedly, # is valid comment for assembly and GHCs hadrian is building libffi just fine. But for the cabal libffi-clib package we seem to run into problems. This must be related to how cabal invokes the preprocessor.

[hasufell]

This was reproduced with the linux docker images from GHC's ci-images https://gitlab.haskell.org/ghc/ci-images

Adding such comments to src/x86/unix64.S and building on my OpenSUSE didn't trigger it.