Disable the ARM & ARM64 UnsafeCASObject intrinsic with heap poisoning.

The current heap poisoning instrumentation of this intrinsic
does not always work properly when heap poisoning in
enabled, hence this quick fix to let the build & test
infrastructure turn green again.

Bug: 12687968
Change-Id: I546a392a61e429cd13209261f806d0aed8d1cd86