spidermonkey: fix i686 error on 38

spidermonkey doesn’t compile correctly on i686 linux & gcc7. It gives
the error:

with gcc-7: undefined reference to `__divmoddi4'

pkgs/top-level/all-packages.nix

···
   spidermonkey_1_8_5 = callPackage ../development/interpreters/spidermonkey/1.8.5.nix { };
   spidermonkey_17 = callPackage ../development/interpreters/spidermonkey/17.nix { };
   spidermonkey_31 = callPackage ../development/interpreters/spidermonkey/31.nix { };
-  spidermonkey_38 = callPackage ../development/interpreters/spidermonkey/38.nix {
+  spidermonkey_38 = callPackage ../development/interpreters/spidermonkey/38.nix ({
     inherit (darwin) libobjc;
-  };
+  } // (stdenv.lib.optionalAttrs (stdenv.cc.isGNU && stdenv.hostPlatform.isi686) {
+      stdenv = overrideCC stdenv gcc6; # with gcc-7: undefined reference to `__divmoddi4'
+  }));
   spidermonkey_52 = callPackage ../development/interpreters/spidermonkey/52.nix { };
   spidermonkey = spidermonkey_31;