Alec Theriault authored Nov 22, 2018 and Ben Gamari committed Nov 22, 2018

* Pass 'GHC/Prim.hs' to Haddock when processing 'ghc-prim'. This
  file is autogenerated for the sole purpose of giving Haddock
  something to process, so we really should make sure it gets
  through to Haddock!

* Add a "docs-haddock" build rule, which should build all Haddock docs
  that the Makefile builds by default (all libs + index for libs + ghc)

* Prune some unnecessary rules (esp. `gen_contents_index`)

Reviewers: bgamari, snowleopard

Reviewed By: snowleopard

Subscribers: alpmestan, snowleopard, rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5316