WIP run-time target for primops

If GHC is going to be multi-target, we cannot choose a target at build time for the primops. Instead, I'll add a basic macro language to genprimop than can generate CPP for haddocking, and conditional code for the *.hs-include files.

• 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:.