Ensure flags destined for ld are properly passed

GHC uses gcc, not ld, for linking. Consequently all flags to be
interpreted by ld need to be prefixed by -optl,-Wl on the GHC command
line.

Test Plan: Validate on OpenBSD

Reviewers: austin, rwbarton

Reviewed By: rwbarton

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2776