archimedes: use latest toolchain, not gcc-6

Without the change archimedes fails to build with latest toolchains as:

ld: archimedes.c:(.text+0xdac7): undefined reference to `rnd'
ld: archimedes.c:(.text+0xdeab): undefined reference to `rnd'
ld: archimedes.c:(.text+0xdf13): undefined reference to `rnd'
ld: archimedes.c:(.text+0xe3ef): undefined reference to `rnd'
ld: archimedes.o:archimedes.c:(.text+0xe456): more undefined references to `rnd' follow
ld: archimedes.o: in function `EMC':
archimedes.c:(.text+0xf11b): undefined reference to `creation'
ld: archimedes.c:(.text+0xf5f5): undefined reference to `creation'
ld: archimedes.c:(.text+0xf826): undefined reference to `creation'
ld: archimedes.c:(.text+0xf9ce): undefined reference to `creation'

The change happens to fix build against clang as well.

pkgs/applications/science/electronics/archimedes/default.nix

···
-{ lib, stdenv, fetchurl }:
+{ lib, stdenv, fetchurl, fetchpatch }:
 
 stdenv.mkDerivation rec {
   pname = "archimedes";
···
     url = "mirror://gnu/archimedes/archimedes-${version}.tar.gz";
     sha256 = "0jfpnd3pns5wxcxbiw49v5sgpmm5b4v8s4q1a5292hxxk2hzmb3z";
   };
+
+  patches = [
+    # Pull patch pending upstream inclusion to support c99 toolchains:
+    #   https://savannah.gnu.org/bugs/index.php?62703
+    (fetchpatch {
+      name = "c99.patch";
+      url = "https://savannah.gnu.org/bugs/download.php?file_id=53393";
+      sha256 = "1xmy1w4ln1gynldk3srdi2h0fxpx465dsa1yxc3rzrrjpxh6087f";
+    })
+  ];
 
   meta = {
     description = "GNU package for semiconductor device simulations";

pkgs/top-level/all-packages.nix

···
 
   astral = callPackage ../applications/science/biology/astral { };
 
-  archimedes = callPackage ../applications/science/electronics/archimedes {
-    stdenv = gcc6Stdenv;
-  };
+  archimedes = callPackage ../applications/science/electronics/archimedes { };
 
   bayescan = callPackage ../applications/science/biology/bayescan { };