fix compilation failure on OpenBSD with system supplied GNU C 4.2.1
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

sdiehl/ghc,olsner/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc,mettekou/ghc,ezyang/ghc,olsner/ghc,sdiehl/ghc,ezyang/ghc,snoyberg/ghc,sdiehl/ghc,sdiehl/ghc,mettekou/ghc,snoyberg/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,mettekou/ghc,olsner/ghc,sdiehl/ghc,snoyberg/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,shlevy/ghc,mettekou/ghc,sdiehl/ghc,snoyberg/ghc,olsner/ghc,mettekou/ghc,snoyberg/ghc,olsner/ghc,mettekou/ghc,olsner/ghc,shlevy/ghc,ezyang/ghc,snoyberg/ghc