always use -fPIC on OpenBSD/AMD64 platform

Summary:
This patch switches -fPIC on for every invocation of GHC
on OpenBSD/AMD64 platform. The reason is OpenBSD's support
for PIE (PIC for executables) hence -fPIC is also needed
for GHC compiled code.

Fixes #10597

Reviewers: austin

Subscribers: thomie, bgamari

Differential Revision: https://phabricator.haskell.org/D1027