Experimental VM flag to enforce access atomicity

In course of JMM 9 work [1], we need to conduct a few performance studies.
This particular study needs to figure out if the access-atomicity exception for
64-bit data types is pragmatic at this point.

This issue tracks the experimental VM work for that; it can be also merged to master,
if we want to simplify performance study freeing everyone from building the patched VM.

[1] http://openjdk.java.net/jeps/188