RFR: 8166862: CMS needs klass_or_null_acquire

[Kim Barrett]

Please review this change to the CMS collector to replace uses of
klass_or_null with klass_or_null_acquire where needed.

[Reviewing on hotspot-dev; only GC code is involved, but some non-GC
folks have expressed interest.]

All non-assert uses of klass_or_null are being replaced with the
acquire variant.  In most cases, the size is being accessed soon
after, and the acquire is needed to ensure order of klass and possible
inline size access.  In a few cases, the need for acquire is less
immediately obvious:

- CompatibleFreeListSpace::block_is_obj - Size access is not lexically
  apparent, but callers access the size or other data ordered with the
  klass.

- MarkFromRootsClosure::do_bit and ParMarkFromRootsClosure::do_bit:
  These perform other operations that need to be properly ordered wrto
  the klass access.

Removed a couple of explict acquire fences that are made redundant by
the use of klass_or_null_acquire.  The latter is preferable, as it
permits the use of platform-specific load_acquire implementations that
might be better than separate load + acquire fence.

Removed a stale comment about permgen and is_parsable.

CR:
https://bugs.openjdk.java.net/browse/JDK-8166862

Webrev:
http://cr.openjdk.java.net/~kbarrett/8166862/webrev.00/

Testing:
JPRT