I saw it in this commit message and wondered if there's more to this story:

1. gcc6 bumps to isl 0.17 only if compiling for RedoxOS. This
   probably means that gcc6 is able to use isl 0.17 everywhere. If
   not, there should have been a comment explaining why RedoxOS is
   special here.

Alright I have a patch for RedoxOS support, and I've added it to the list of patches for gcc here: https://github.com/NixOS/nixpkgs/compare/master...zmitchell:nixpkgs:zmitchell/redox-gcc-support

I'm not quite sure how to verify that I'm building it properly because I would expect to have to rebuild gcc from source, but a nix-build -A gcc or nix-build -A gcc13 in my nixpkgs fork takes about 2s and seems to substitute gcc rather than build it.

Trying to build the cross-compiler doesn't evaluate either:

$ nix-build -A pkgsCross.x86_64-unknown-redox.gcc13
...
       … from call site

         at /home/zmitchell/nixpkgs/pkgs/top-level/all-packages.nix:14930:16:

        14929|   gcc6Stdenv = overrideCC gccStdenv buildPackages.gcc6;
        14930|   gcc7Stdenv = overrideCC gccStdenv buildPackages.gcc7;
             |                ^
        14931|   gcc8Stdenv = overrideCC gccStdenv buildPackages.gcc8;

       … while calling 'overrideCC'

         at /home/zmitchell/nixpkgs/pkgs/stdenv/adapters.nix:35:24:

           34|   # Override the compiler in stdenv for specific packages.
           35|   overrideCC = stdenv: cc: stdenv.override {
             |                        ^
           36|     allowedRequisites = null;

       error: infinite recursion encountered

       at «none»:0: (source not available)

A cross compiler would be pkgsCross.x86_64-unknown-redox.stdenv.cc or pkgsCross.x86_64-unknown-redox.buildPackages.gcc13