ghc: Fix regression in unregisterised builds

[AlexandreTunstall]

The default GCC compiler in Nixpkgs was recently changed, bringing in a new default C standard (gnu23), which GHC's C backend is not compatible with.
This fix adds a flag to force GCC to use an older compatible C standard.

Things done

• Built on platform:
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• Tested, as applicable:
  • [NixOS tests] in [nixos/tests].
  • [Package tests] at passthru.tests.
  • Tests in [lib/tests] or [pkgs/test] for functions and "core" functionality.
• Ran nixpkgs-review on this PR. See [nixpkgs-review usage].
• Tested basic functionality of all binary files, usually in ./result/bin/.
• Nixpkgs Release Notes
  • Package update: when the change is major or breaking.
• NixOS Release Notes
  • Module addition: when adding a new NixOS module.
  • Module update: when the change is significant.
• Fits [CONTRIBUTING.md], [pkgs/README.md], [maintainers/README.md] and other READMEs.

[sternenseemann]

When you get a minute, it would be interesting to check whether upstream has already looked into fixing the C output for later versions of GHC.

[AlexandreTunstall]

When you get a minute, it would be interesting to check whether upstream has already looked into fixing the C output for later versions of GHC.

This is a good point, as we may want the same fix for some of the Hadrian-based GHCs, which I have not yet looked into.

[AlexandreTunstall]

All Hadrian versions fail unregisterised, so they too need this fix.

However, I will leave that out of this PR.