Darwin, Arm64 : Initial support for the self-host driver.

At present, this just includes the generic Darwin stuff.

gcc/config.host

@@ -99,7 +99,8 @@ case ${host} in
 esac
 
 case ${host} in
-  aarch64*-*-freebsd* | aarch64*-*-linux* | aarch64*-*-fuchsia*)
+  aarch64*-*-freebsd* | aarch64*-*-linux* | aarch64*-*-fuchsia* |\
+  aarch64-*-darwin* | arm64*-*-darwin*)
     case ${target} in
       aarch64*-*-*)
 	host_extra_gcc_objs="driver-aarch64.o"
@@ -251,6 +252,10 @@ case ${host} in
     host_extra_gcc_objs="${host_extra_gcc_objs} driver-mingw32.o"
     host_lto_plugin_soname=liblto_plugin.dll
     ;;
+  aarch64-*-darwin* | arm64-*-darwin*)
+    out_host_hook_obj="${out_host_hook_obj} host-aarch64-darwin.o"
+    host_xmake_file="${host_xmake_file} aarch64/x-darwin"
+    ;;
   i[34567]86-*-darwin* | x86_64-*-darwin*)
     out_host_hook_obj="${out_host_hook_obj} host-i386-darwin.o"
     host_xmake_file="${host_xmake_file} i386/x-darwin"

gcc/config/aarch64/host-aarch64-darwin.c

@@ -0,0 +1,32 @@
+/* Arm64-darwin host-specific hook definitions.
+   Copyright (C) 2020 Free Software Foundation, Inc.
+
+This file is part of GCC.
+
+GCC is free software; you can redistribute it and/or modify it under
+the terms of the GNU General Public License as published by the Free
+Software Foundation; either version 3, or (at your option) any later
+version.
+
+GCC is distributed in the hope that it will be useful, but WITHOUT ANY
+WARRANTY; without even the implied warranty of MERCHANTABILITY or
+FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
+for more details.
+
+You should have received a copy of the GNU General Public License
+along with GCC; see the file COPYING3.  If not see
+<http://www.gnu.org/licenses/>.  */
+
+#define IN_TARGET_CODE 1
+
+#include "config.h"
+#include "system.h"
+#include "coretypes.h"
+#include "hosthooks.h"
+#include "hosthooks-def.h"
+#include "config/host-darwin.h"
+
+/* Darwin doesn't do anything special for arm64/aarch64 hosts; this file
+   exists just to include the generic config/host-darwin.h.  */
+
+const struct host_hooks host_hooks = HOST_HOOKS_INITIALIZER;

gcc/config/aarch64/x-darwin

@@ -0,0 +1,3 @@
+host-aarch64-darwin.o : $(srcdir)/config/aarch64/host-aarch64-darwin.c
+	$(COMPILE) $<
+	$(POSTCOMPILE)