RFR: 8247524: Remove unnecessary volatile qualifiers from member functions

[Kim Barrett]

Please review this change to oopDesc and Metadata classes, removing
unnecessary volatile qualifiers from some member functions.

This change also makes virtual overrides of the relevant functions
explicitly virtual.

While there, removed unnecessary workaround in oopDesc::klass_or_null_acquire.

After this change,
  egrep -r -H --exclude-dir=.hg "\)\s*(const |)\s*volatile" src/hotspot
finds nothing; there seem to be no remaining volatile qualified methods.

CR:
https://bugs.openjdk.java.net/browse/JDK-8247524

Webrev:
https://cr.openjdk.java.net/~kbarrett/8247524/open.00/

Testing:
mach5 tier1-3