overlay: do not override LLVM for non-Genode

overlay/default.nix

@@ -158,9 +158,9 @@ in nullPkgs // {
     } else
       linuxPackages;
 
-  llvmPackages_11 =
-    # A copy of the LLVM expressions from Nixpkgs.
-    # Nothing here is upstreamed.
+  llvmPackages_11 = if targetPlatform.isGenode then
+  # A copy of the LLVM expressions from Nixpkgs.
+  # Nothing here is upstreamed.
     callPackage ./llvm-11 ({
       inherit (stdenvAdapters) overrideCC;
       buildLlvmTools = buildPackages.llvmPackages_11.tools;
@@ -168,7 +168,9 @@ in nullPkgs // {
     } // lib.optionalAttrs
       (stdenv.hostPlatform.isi686 && buildPackages.stdenv.cc.isGNU) {
         stdenv = gcc7Stdenv;
-      });
+      })
+  else
+    prev.llvmPackages_11;
 
   ncurses =
     # https://invisible-island.net/autoconf/