X86: Implement VarHandle.getAndAdd intrinsic

This commit implements VarHandle getAndAdd intrinsic. This also implied
adding xadd instruction and tests for it.

Test: art/test.py --host -r -t 712-varhandle-invocation --32
Test: m test-art-host-gtest
Bug: 65872996
Change-Id: I84dd95ba6464c8a73ace03a13817147c7099677a
5 files changed