The ghc.mk file is generated by `./boot` and should be excluded from git
via .gitignore, since the file `ghc.mk` may have different line ends on
windows.

The file GNUmakefile and dir dist, dist-install should also be excluded
via .gitignore, just as other libraries.

Test Plan: [skip ci]

Reviewers: bgamari

Reviewed By: bgamari

Subscribers: osa1, rwbarton, thomie, carter

Differential Revision: https://phabricator.haskell.org/D4814