RFR(S): 8186611: s390: Add missing compiler barriers and fix assembler

[Doerr, Martin]

Hi,

please review my small s390 fix.
As discovered by Kim Barret, the current inline assembly implementation of the atomics lacks "memory" in the clobber lists. This may lead to undesired compiler optimizations. We have never observed an issue, but it should be fixed.
Another bug is in the assembler encoding of the mvh* instructions which take and Address parameter.
In addition, I've improved an assertion which prevents the compiler from generating multiple accesses.

Webrev:
http://cr.openjdk.java.net/~mdoerr/8186611_s390_fixes/webrev.00/

Thanks and best regards,
Martin