Draft: Always build GHCi, using iserv if needed

Rebase of !1405 (closed)

This builds GHCi under all conditions, by allowing fexternal-interpreter to work when he internal interpreter isn't built.

Continues the fight where !1390 (closed) left off.

I tested that this works for basic things like 1 + 1, but we would want to modify the testsuite so that we can properly test the ghc built in stage0. It's quite likely that some CPPing ought to be added within the now-un-CPPed code, so we are affectively pushing the CPP deeper rather than removing it entirely.

CC @alp @angerman @hvr since you all are CC'd in !1390 (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:.