ARM64: Reimplement the UnsafeCASObject intrinsic.

For the UnsafeCASObject with Baker read barriers, drop the
old code updating the field. Perform the main path CAS loop
and redirect the flow for failure to a slow path that marks
the old value and compares it with the expected value (if
not marking, this is just a few instructions to determine
that they differ). If it's the same, the old value is known
to be the from-space reference to the expected object and
the slow path performs a modified CAS loop checking for both
expected object references (from-space and to-space).

Test: Already covered by the 160-read-barrier-stress.
Test: m test-art-host-gtest
Test: Pixel 2 XL boots.
Test: testrunner.py --target --optimizing --64
Test: ART_HEAP_POISONING=true testrunner.py --target --optimizing --64
Bug: 36141117
Change-Id: I175806dbc003640c9bb6759be6788311bcc9310c

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