configure: Accept suffix in OpenBSD triple's OS name
configure: Accept suffix in OpenBSD triple's OS name

Reviewers: hvr

Subscribers: rwbarton, thomie, erikd, carter

Differential Revision: https://phabricator.haskell.org/D4483

sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc