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