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/services/std_svc/sdei/sdei_private.h b/services/std_svc/sdei/sdei_private.h
index 8cc66e7..44a7301 100644
--- a/services/std_svc/sdei/sdei_private.h
+++ b/services/std_svc/sdei/sdei_private.h
@@ -22,7 +22,7 @@
 #include <services/sdei.h>
 #include <setjmp.h>
 
-#ifdef AARCH32
+#ifndef __aarch64__
 # error SDEI is implemented only for AArch64 systems
 #endif