darwin.cctools: add a gas output

The ld64 upgrade moves gas to a separate output. Prepare for the update by moving gas in cctools-port to the output.

pkgs/os-specific/darwin/cctools/port.nix

@@ -33,7 +33,7 @@ stdenv.mkDerivation {
     sha256 = "0ns12q7vg9yand4dmdsps1917cavfbw67yl5q7bm6kb4ia5kkx13";
   };
 
-  outputs = [ "out" "dev" "man" ];
+  outputs = [ "out" "dev" "gas" "man" ];
 
   nativeBuildInputs = [ autoconf automake libtool autoreconfHook installShellFiles ]
     ++ lib.optionals (stdenv.isDarwin && stdenv.isx86_64) [ memstreamHook ];
@@ -178,6 +178,12 @@ stdenv.mkDerivation {
     popd
   '';
 
+  postInstall = ''
+    # Move GNU as to its own output to prevent it from being used accidentally.
+    moveToOutput bin/gas "$gas"
+    moveToOutput libexec "$gas"
+  '';
+
   passthru = {
     inherit targetPrefix;
   };