Delete __GLASGOW_HASKELL__ ifdefs for stage0 < 7.6.

Summary:
My understanding is that ghc 7.10 should be buildable with the last 3 versions
of ghc, i.e 7.6, 7.8 and 7.10 itself.

Test Plan: x

Reviewers: austin

Reviewed By: austin

Subscribers: hvr, simonmar, ezyang, carter, thomie

Differential Revision: https://phabricator.haskell.org/D254