Refactoring and enhancements to RGBASM warnings (#1526)

* Allow a `no-` prefix to negate "meta" warnings
  (`-Wno-all`, `-Wno-extra`, `-Wno-everything`)
* Allow `-Wno-error=...` to override `-Werror`
  (including for "meta" warnings)

src/asm/main.cpp

@@ -293,7 +293,7 @@ int main(int argc, char *argv[]) {
 			break;
 
 		case 'W':
-			processWarningFlag(musl_optarg);
+			opt_W(musl_optarg);
 			break;
 
 		case 'w':