X86: Add tests for cmpxchgw and cmpxchgb

The two instructions were added in a previous commit and were needed for
VarHandle.compareAndSet intrinsic.

Test: m art_compiler_host_tests
Bug: 65872996
Change-Id: If2a9d8d1a4c40034d16e2082f2e5948bcdfbab21
2 files changed