fix compilation failure on OpenBSD with system supplied GNU C 4.2.1

Authored by kgardas on Aug 13 2016, 11:46 AM.

Description

fix compilation failure on OpenBSD with system supplied GNU C 4.2.1

Summary:
This patch fixes compilation failure on OpenBSD. The OpenBSD's
GNU C compiler is of 4.2.1 version and problematic builtin_unreachable
was added in GNU C 4.5 release. Let's use pure abort() call
on OpenBSD instead of
builtin_unreachable

Reviewers: bgamari, austin, erikd, simonmar

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2453