Create RTS configure script that is responsible for ghcautoconf.h and ghcplatform.h

There are still other aspects of the top-level configure script that are RTS specific and ought to be moved over, but this does the bulk of the work.

Progress towards #17191 (closed). Rewrite of !1897 (closed).

Please take a few moments to verify that your commits fulfill the following:

• 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:.