Switch AARCH32/AARCH64 to __aarch64__

NOTE: AARCH32/AARCH64 macros are now deprecated in favor of __aarch64__.

All common C compilers pre-define the same macros to signal which
architecture the code is being compiled for: __arm__ for AArch32 (or
earlier versions) and __aarch64__ for AArch64. There's no need for TF-A
to define its own custom macros for this. In order to unify code with
the export headers (which use __aarch64__ to avoid another dependency),
let's deprecate the AARCH32 and AARCH64 macros and switch the code base
over to the pre-defined standard macro. (Since it is somewhat
unintuitive that __arm__ only means AArch32, let's standardize on only
using __aarch64__.)

Change-Id: Ic77de4b052297d77f38fc95f95f65a8ee70cf200
Signed-off-by: Julius Werner <jwerner@chromium.org>
diff --git a/include/lib/xlat_tables/xlat_tables_defs.h b/include/lib/xlat_tables/xlat_tables_defs.h
index 000811f..76cfc0b 100644
--- a/include/lib/xlat_tables/xlat_tables_defs.h
+++ b/include/lib/xlat_tables/xlat_tables_defs.h
@@ -62,7 +62,7 @@
 #define OSH			(U(0x2) << 6)
 #define ISH			(U(0x3) << 6)
 
-#ifdef AARCH64
+#ifdef __aarch64__
 /* Guarded Page bit */
 #define GP			(ULL(1) << 50)
 #endif