RFR: 8166583: Add oopDesc::klass_or_null_acquire()

[Kim Barrett]

Please review this addition of a new function to oopDesc.  This is a
variant of an existing function; the new function provides acquire
semantics to the access.

This is a subtask of JDK-8160369, providing additional infrastructure
needed to fix that bug.

CR:
https://bugs.openjdk.java.net/browse/JDK-8166583

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166583/webrev.00/

Testing:
Ran nightly tests with this being used by an in-development fix for
part of JDK-8160369.