Optimize casMutVar# for single-threaded RTS
Optimize casMutVar# for single-threaded RTS

The single-threaded RTS shouldn't actually need to use CAS to
implement `casMutVar#`; there are no other threads to coordinate
with.

Reviewers: austin, bgamari, erikd, simonmar

Reviewed By: simonmar

Subscribers: rwbarton, thomie

Differential Revision: https://phabricator.haskell.org/D3544

sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,sdiehl/ghc,sdiehl/ghc,ezyang/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,shlevy/ghc