Guard inclusion of Availability.h so that it only happens on arm where it's
actuallty used so it doesn't cause failures on old OS X where Availability.h
doesn't exist yet.

Michael Weiser <michael@weiser.dinsnail.net>

--- Csu-85/start.s.orig	2013-10-09 01:11:47.000000000 +0200
+++ Csu-85/start.s	2015-02-11 20:34:04.000000000 +0100
@@ -22,7 +22,9 @@
  * @APPLE_LICENSE_HEADER_END@
  */
 
+#ifdef __arm__
 #include <Availability.h>
+#endif
 
 #if __ppc__ && __DYNAMIC__
 //