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

(cherry picked from commit d40d6df74a9fca382a84709ac9bc862897fbec18)

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