It still errors with infinite recursion. I have it setup as

highPower = with armPkgs;
  callPackage ./default.nix {
    stdenv = 
      overrideCC stdenv (buildPackages.wrapCC (stdenv.cc
        // {
          stdenv.cc.targetPlatform =
            stdenv.cc.targetPlatform
            // {
              gcc =
                stdenv.cc.targetPlatform.gcc
                // {
                  arch = "arv6m";
                  cpu = "cortex-m0";
                  thumb = true;
                };
            };
        }));
  };