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