Deduplicate `HaskellMachRegs.h` and `RtsMachRegs.h` headers

Until 0472f0f6 there was a meaningful host vs target distinction (though it wasn't used right, in genapply). After that, they did not differ in meaningful ways, so it's best to just only keep one.

• 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:.