StgCmmPrim: Add missing MO_WriteBarrier
StgCmmPrim: Add missing MO_WriteBarrier

Test Plan: Good question

Reviewers: austin, trommler, simonmar, rrnewton

Reviewed By: simonmar

Subscribers: RyanGlScott, thomie

Differential Revision: https://phabricator.haskell.org/D2495

GHC Trac Issues: #12469

mettekou/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,snoyberg/ghc,olsner/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,mettekou/ghc,snoyberg/ghc,snoyberg/ghc,mettekou/ghc,mettekou/ghc,ezyang/ghc,snoyberg/ghc,mettekou/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,mettekou/ghc,olsner/ghc,olsner/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,snoyberg/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,olsner/ghc,ezyang/ghc,sdiehl/ghc,olsner/ghc,shlevy/ghc,snoyberg/ghc,ezyang/ghc,snoyberg/ghc,sdiehl/ghc,olsner/ghc