gcc6, gcc7, gcc8, gcc9, gcc10, gcc11: do not modify code in prePatch

[bobrik]

Motivation for this change

Doing any modifications before patching risks invalidating patch base.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option sandbox in nix.conf on non-NixOS linux)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Determined the impact on package closure size (by running nix path-info -S before and after)
• Ensured that relevant documentation is up to date
• Fits CONTRIBUTING.md.

[vcunat]

Well, simple prePatch = ""; would avoid the -linux rebuilds (gcc9 for aarch64, gcc10 for others), but I expect the change doesn't hurry because it just addresses some hypothetical future situation.

[bobrik]

#115235 is not hypothetical.

[vcunat]

Ah, I missed that. The x86_64-darwin rebuilds would force this through staging anyway. I'm actually surprised that gcc can cause so many rebuilds in there, with stdenv being clang-only.

[bobrik]

I think numpy (which depends on gfortran) is the root cause of many changes for x86_64-darwin.