gcc*: force building a cross compiler if host and target platform differ

[sternenseemann]

gcc's configure system has the nasty habit (for us) of judging for
itself if it is building a cross compiler (or cross compiling), but on
the limited information of the build, host and target platforms' config
which only contains a subset of the information we encode in
stdenv.*Platform. The practical consequence was that prior to this
change building pkgsLLVM.buildPackages.gcc actually fails because it
refuses to use --with-headers with something it believes to not be a
cross compiler.

As a workaround we force the appropriate variable in the configure
script to always be yes regardless of its own conditional check.

At some point we probably should report this issue in some capacity, so
future gcc versions don't force us into workarounds like this and
acdc783.

Now the following works which failed pretty early previously (on staging):

nix-build -A pkgsLLVM.buildPackages.gcc
/nix/store/v3rrxn542209lqzwb4yhyj6zwi1shskb-x86_64-unknown-linux-gnu-stage-final-gcc-wrapper-10.3.0

Motivation for this change

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.

[Ericson2314]

Nice! I had to refresh myself as to why the other GCC prefix PR didn't do the trick, but it makes total sense now that I read both. Thanks!

[sternenseemann]

Nice! I had to refresh myself as to why the other GCC prefix PR didn't do the trick, but it makes total sense now that I read both. Thanks!

Exactly, the other PR did fix gccCrossStageStatic which did not pass --with-headers in the first place, so the issue fixed here never occured.