pass -z wxneeded or -Wl,-zwxneeded for linking on OpenBSD
pass -z wxneeded or -Wl,-zwxneeded for linking on OpenBSD

Summary:
This patch fixes issue with abort in GHCi on OpenBSD current
as of Aug 12 2016. The OpenBSD is more and more strict about usage
of writable and executable memory. Programs/applications which
requires such functionality need to be linked with -z wxneeded linker
flag and need to be run from the file-system mounted with wxallowed
mount option. If either of those options in not met, then problematic
program/application usually fail on some mmap/mprotect call which fail.

Reviewers: bgamari, austin, hvr

Subscribers: thomie, erikd

Differential Revision: https://phabricator.haskell.org/D2454

(cherry picked from commit f9aa996f0af59f32dc7b1528ff78be41413a9c27)

GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc,GaloisInc/halvm-ghc