tree-wide: use gcc attribute macros where appropriate

We have these macros already, hence use them.

src/basic/util.h

@@ -173,7 +173,7 @@ static inline void _reset_errno_(int *saved_errno) {
 }
 
 #define PROTECT_ERRNO                                                   \
-        _cleanup_(_reset_errno_) __attribute__((__unused__)) int _saved_errno_ = errno
+        _cleanup_(_reset_errno_) _unused_ int _saved_errno_ = errno
 
 static inline int negative_errno(void) {
         /* This helper should be used to shut up gcc if you know 'errno' is