RFR: AArch64: 8179954: AArch64: C1 and C2 volatile accesses are not sequentially consistent

[Andrew Haley]

Thanks for finding that bug.  It's a reminder that we need to
concentrate on doing the minimum at this stage to ensure correctness.
Our performance is good, and this change is not hugely profitable.  It's
not worth risking the whole ship for.

Nevertheless, I've made a new webrev, which does the right thing.  I
stepped through the code to make sure.  I've come this far, so I might
as well get it right.  (Yes, I'm aware that I just fell into the sunk
cost fallacy, but I want something to show for the work I've done.)

http://cr.openjdk.java.net/~aph/8179954-3/

In JDK 10 we should look at replacing all the explicit fences used for
volatiles with LDAR/STLR.

OK?  I'd like two reviewers for this one.

Andrew.