Modifications to support loading GHC into GHCi

This change was previously part of
[D4904](https://phabricator.haskell.org/D4904), but is being split off
to aid in getting this reviewed and merged.

* The compiler code is built with `NoImplicitPrelude`, but GHCi's
  modules are incompatible with it. So, this adds the pragma to all GHCi
  modules that didn't have it, and adds imports of Prelude.

* In order to run GHC within itself, a `call of 'initGCStatistics`
  needed to be skipped. This uses CPP to skip it when
  `-DGHC_LOADED_INTO_GHCI` is set.

* There is an environment variable workaround suggested by Ben Gamari
  [1], where `_GHC_TOP_DIR` can be used to specify GHC's top dir if `-B`
  isn't provided.  This can be used to solve a problem where the GHC being
  run within GHCi attempts to look in `inplace/lib/lib/` instead of
  `inplace/lib/`.

[1]: https://phabricator.haskell.org/D4904#135438

Reviewers: goldfire, bgamari, erikd, alpmestan

Reviewed By: alpmestan

Subscribers: alpmestan, lelf, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4986

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