refactor test for __builtin_unreachable into Rts.h macro RTS_UNREACHABLE
Summary:
This patch refactors GNU C version test (for 4.5 and more modern)
due to usage of __builtin_unreachable done in the CNF.c code directly
into the new RTS_UNREACHABLE macro placed into Rts.h
Reviewers: bgamari, austin, simonmar, erikd
Subscribers: thomie
Differential Revision: https://phabricator.haskell.org/D2457