RFR: 8217576: C1 atomic access handlers use incorrect decorators

[Kim Barrett]

Please review this fix of decorator defaulting in C1 atomic access
generators.  As noted in the bug, C1 presently ignores the decorators
and only generates fully barriered operations, so the incorrect
decorators haven't been affecting code generation.

CR:
https://bugs.openjdk.java.net/browse/JDK-8217576

Webrev:
http://cr.openjdk.java.net/~kbarrett/8217576/open.00/

Testing:
mach5 tier1.