x86_64: Add LockXadd* helper function and tests for it.

A separate function is need for 16-bit variant `lock xaddw` because the
LOCK prefix should go after the operand size override prefix to match
the order of prefixes emitted by the reference implementation.

Bug: 65872996
Test: m test-art-host-gtest  # new test cases for LOCK XADD
Change-Id: I29946c75c76966cb9c6425b2a22eae3664ce3c00