gnat{11,12,13}: fix build on x86_64-darwin

- gnat11: make sure to use the gnat-bootstrap gcc in the stdenv; and
- Drop the dual assemblers. x86_64-darwin uses the clang assembler by
default, so it no longer needs the workaround for the GNU assembler.

pkgs/development/compilers/gcc/common/pre-configure.nix

···
   export GFORTRAN_FOR_TARGET=${pkgsBuildTarget.gfortran}/bin/${stdenv.targetPlatform.config}-gfortran
 ''
 
-# On x86_64-darwin, the gnat-bootstrap bootstrap compiler that we need to build a
-# native GCC with Ada support emits assembly that is accepted by the Clang
-# integrated assembler, but not by the GNU assembler in cctools-port that Nix
-# usually in the x86_64-darwin stdenv.  In particular, x86_64-darwin gnat-bootstrap
-# emits MOVQ as the mnemonic for quadword interunit moves, such as between XMM
-# and general registers (e.g "movq %xmm0, %rbp"); the cctools-port assembler,
-# however, only recognises MOVD for such moves.
-#
-# Therefore, for native x86_64-darwin builds that support Ada, we have to use
-# the Clang integrated assembler to build (at least stage 1 of) GCC, but have to
-# target GCC at the cctools-port GNU assembler.  In the wrapped x86_64-darwin
-# gnat-bootstrap, the former is provided as `as`, while the latter is provided as
-# `gas`.
-#
-+ lib.optionalString (
-    langAda
-    && buildPlatform == hostPlatform
-    && hostPlatform == targetPlatform
-    && targetPlatform.isx86_64
-    && targetPlatform.isDarwin
-  ) ''
-  export AS_FOR_BUILD=${gnat-bootstrap}/bin/as
-  export AS_FOR_TARGET=${gnat-bootstrap}/bin/gas
-''
-
 # NOTE 2020/3/18: This environment variable prevents configure scripts from
 # detecting the presence of aligned_alloc on Darwin.  There are many facts that
 # collectively make this fix necessary:

pkgs/top-level/all-packages.nix

···
          && stdenv.buildPlatform == stdenv.hostPlatform
       then buildPackages.gnat-bootstrap11
       else buildPackages.gnat11;
+    stdenv =
+      if stdenv.hostPlatform == stdenv.targetPlatform
+         && stdenv.buildPlatform == stdenv.hostPlatform
+         && stdenv.buildPlatform.isDarwin
+         && stdenv.buildPlatform.isx86_64
+      then overrideCC stdenv gnat-bootstrap11
+      else stdenv;
   });
 
   gnat12 = wrapCC (gcc12.cc.override {