Currently, the linux stdenv has

extraAttrs = {
      inherit (stage4.pkgs) glibc;
      ...
};

which gives it a glibc attribute, but now that we're getting a proper darwin stdenv, it would be nice to be able to refer to its standard library generically. Darwin's stdenv already has a libc in its extraAttrs, so I think the solution to this would be as simple as putting that attribute into the linux stdenv's extraAttrs, or into the generic stdenv builder. Adding attributes to the various bootstrap phases will cause a massive linux rebuild though...