Add more read barriers to the class linker.

This change makes it possible to concurrently scan the remaining roots
in the class linker (the non-class-table roots that are visited by
ClassLinker::VisitRoots()) by adding read barriers.

Bug: 12687968
Change-Id: I66fecf7a303eee7537429e018f38da8270b18c67
4 files changed