Fix a latent bug with parsing macro args

This seems to only have worked by coincidence; `$$` was an
undefined value that happened to equal `$1` already.

src/asm/parser.y

@@ -525,7 +525,8 @@ macroargs:
 			fatalerror("Failed to allocate memory for macro arguments: %s\n", strerror(errno));
 	}
 	| macroargs T_STRING {
-		$$->append($2);
+		$1->append($2);
+		$$ = $1;
 	}
 ;