Fix (non-intrinsic) UnsafeCASObject for the read barrier config.

Make sure the field contains a to-space reference before attempting the
CAS with a special read barrier to avoid an incorrect CAS failure.

This is only about the non-intrinsic UnsafeCASObject.

This seems to fix some jsr166 test failures.

Also, remove the unused template parameter kMaybeDuringStartup.

Bug: 25883050
Bug: 12687968

Change-Id: Ia6f0d882fa3d90c42f14968672d547babcdf6309
5 files changed