Remove read barrier in UnstartedUnsafeCompareAndSwapLong.

art::interpreter::UnstartedRuntime::UnstartedUnsafeCompareAndSwapLong
manipulates a long field and does not need a read barrier
(read barriers are only required when loading an object
reference from the heap).

Test: ART_USE_READ_BARRIER=true make test-art-host
Bug: 12687968
Change-Id: Ia46590e649a7bb46ca4b3c90bab03fb3a836563d