gcc: native aarch64-darwin support

pkgs/development/compilers/gcc/11/default.nix

@@ -72,6 +72,11 @@ let majorVersion = "11";
       ++ optional langFortran ../gfortran-driving.patch
       ++ optional (targetPlatform.libc == "musl" && targetPlatform.isPower) ../ppc-musl.patch
 
+      ++ optional (stdenv.isDarwin && stdenv.isAarch64) (fetchpatch {
+        url = "https://github.com/fxcoudert/gcc/compare/releases/gcc-11.1.0...gcc-11.1.0-arm-20210504.diff";
+        sha256 = "sha256-JqCGJAfbOxSmkNyq49aFHteK/RFsCSLQrL9mzUCnaD0=";
+      })
+
       # Obtain latest patch with ../update-mcfgthread-patches.sh
       ++ optional (!crossStageStatic && targetPlatform.isMinGW) ./Added-mcf-thread-model-support-from-mcfgthread.patch;
 
@@ -103,9 +108,15 @@ stdenv.mkDerivation ({
 
   hardeningDisable = [ "format" "pie" ];
 
+  postPatch = ''
+    configureScripts=$(find . -name configure)
+    for configureScript in $configureScripts; do
+      patchShebangs $configureScript
+    done
+  ''
   # This should kill all the stdinc frameworks that gcc and friends like to
   # insert into default search paths.
-  prePatch = lib.optionalString hostPlatform.isDarwin ''
+  + lib.optionalString hostPlatform.isDarwin ''
     substituteInPlace gcc/config/darwin-c.c \
       --replace 'if (stdinc)' 'if (0)'
 
@@ -114,14 +125,8 @@ stdenv.mkDerivation ({
 
     substituteInPlace libgfortran/configure \
       --replace "-install_name \\\$rpath/\\\$soname" "-install_name ''${!outputLib}/lib/\\\$soname"
-  '';
-
-  postPatch = ''
-    configureScripts=$(find . -name configure)
-    for configureScript in $configureScripts; do
-      patchShebangs $configureScript
-    done
-  '' + (
+  ''
+  + (
     if targetPlatform != hostPlatform || stdenv.cc.libc != null then
       # On NixOS, use the right path to the dynamic linker instead of
       # `/lib/ld*.so'.

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

@@ -170,7 +170,8 @@ let
     ++ lib.optional javaAwtGtk "--enable-java-awt=gtk"
     ++ lib.optional (langJava && javaAntlr != null) "--with-antlr-jar=${javaAntlr}"
 
-    ++ (import ../common/platform-flags.nix { inherit (stdenv)  targetPlatform; inherit lib; })
+    # TODO: aarch64-darwin has clang stdenv and its arch and cpu flag values are incompatible with gcc
+    ++ lib.optional (!(stdenv.isDarwin && stdenv.isAarch64)) (import ../common/platform-flags.nix { inherit (stdenv)  targetPlatform; inherit lib; })
     ++ lib.optionals (targetPlatform != hostPlatform) crossConfigureFlags
     ++ lib.optional (targetPlatform != hostPlatform) "--disable-bootstrap"