Use non-CAS thread flip root visitor.

We don't need to use CAS to update the thread-local GC roots for the
thread flip.

Bug: 12687968
Bug: 29517059
Test: libartd.so boot. ART tests. Ritzperf EAAC.
Change-Id: Ia2acab824f756bd7d2ad501b2040233e0d394356
1 file changed