fix darwin threaded static linking by removing -lpthread option #9189

Summary: Signed-off-by: Bob Ippolito <bob@redivi.com>

Test Plan: See repro instructions in trac #9189

Reviewers: austin

Reviewed By: austin

Subscribers: phaskell, simonmar, relrod, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D120

GHC Trac Issues: #9189