Remove direct usage of __attribute__((foo))

Migrate all direct usage of __attribute__ to usage of their
corresponding macros from cdefs.h.
e.g.:
 - __attribute__((unused)) -> __unused

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
diff --git a/include/lib/cassert.h b/include/lib/cassert.h
index e8089cb..00ee4d5 100644
--- a/include/lib/cassert.h
+++ b/include/lib/cassert.h
@@ -40,6 +40,6 @@
  * compiler warning.
  ******************************************************************************/
 #define CASSERT(cond, msg)	\
-	typedef char msg[(cond) ? 1 : -1] __attribute__((unused))
+	typedef char msg[(cond) ? 1 : -1] __unused
 
 #endif /* __CASSERT_H__ */