RFR(M): 8202080: Introduce ordering semantics for Atomic::add

[Doerr, Martin]

Hi,

JDK-8195099 "Concurrent safe-memory-reclamation mechanism" introduced a usage of Atomic::add for which we're not using the correct memory barriers on PPC64 and s390.
Some more description can be found in the bug: https://bugs.openjdk.java.net/browse/JDK-8202080

We can fix this by the following change:
http://cr.openjdk.java.net/~mdoerr/8202080_atomic_add/webrev.00/

Please note that I only implemented the platform code for linux x86, PPC64 and s390 so far. The remaining ones should be trivial and I'll take care of them after I got some feedback.
I can also do the same change for other atomic functions like sub, inc and dec if you like.

Intention of this change is only to fix this new code and possibly other future enhancements, not to change the behavior of any older usages of Atomic::add.

Feedback is welcome.

Best regards,
Martin