Fix compile failure on MacOSX

MacOSX tool chain defines __unused in "sys/cdefs.h", causing a collision.
Remove the local define to avoid polluting the compilers internal 
symbol namespace.

Change-Id: I16370c4518e6aeec482dd689e7db80628f846ee3
Signed-off-by: Matthias Welwarsky <matthias.welwarsky@sysgo.com>
Reviewed-on: http://openocd.zylin.com/4118
Reviewed-by: Steven Stallion <sstallion@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Tested-by: jenkins

src/target/aarch64.c

@@ -30,8 +30,6 @@
 #include "armv8_cache.h"
 #include <helper/time_support.h>
 
-#define __unused __attribute((unused))
-
 enum restart_mode {
 	RESTART_LAZY,
 	RESTART_SYNC,