darwin.binutilsDualAs: use gas output of darwin.cctools

pkgs/top-level/darwin-packages.nix

@@ -95,17 +95,29 @@ impure-cmds // appleSourcePackages // chooseLibs // {
     bintools = self.binutils-unwrapped;
   };
 
-  binutilsDualAs-unwrapped = callPackage ../os-specific/darwin/binutils {
+  # x86-64 Darwin gnat-bootstrap emits assembly
-    inherit (pkgs) binutils-unwrapped;
+  # with MOVQ as the mnemonic for quadword interunit moves
-    inherit (pkgs.llvmPackages) llvm clang-unwrapped;
+  # such as `movq %rbp, %xmm0`.
-    dualAs = true;
+  # The clang integrated assembler recognises this as valid,
+  # but unfortunately the cctools.gas GNU assembler does not;
+  # it instead uses MOVD as the mnemonic.
+  # The assembly that a GCC build emits is determined at build time
+  # and cannot be changed afterwards.
+  #
+  # To build GNAT on x86-64 Darwin, therefore,
+  # we need both the clang _and_ the cctools.gas assemblers to be available:
+  # the former to build at least the stage1 compiler,
+  # and the latter at least to be detectable
+  # as the target for the final compiler.
+  binutilsDualAs-unwrapped = pkgs.buildEnv {
+    name = "${lib.getName self.binutils-unwrapped}-dualas-${lib.getVersion self.binutils-unwrapped}";
+    paths = [
+      self.binutils-unwrapped
+      (lib.getOutput "gas" self.cctools)
+    ];
   };
 
-  binutilsDualAs = pkgs.wrapBintoolsWith {
+  binutilsDualAs = self.binutils.override {
-    libc =
-      if stdenv.targetPlatform != stdenv.hostPlatform
-      then pkgs.libcCross
-      else pkgs.stdenv.cc.libc;
     bintools = self.binutilsDualAs-unwrapped;
   };