hadrian: Fix generation of settings

I jumbled some lines in e529c65e, messing up the leading underscores and rts ways settings. This broke at least stage1 linking on macOS, but probably loads of other things too.

Should fix #16685 (closed) and #16658 (closed).

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• replace this message with a description motivating your change

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.