RFR: 8165808: Add release barriers when allocating objects with concurrent collection

[Kim Barrett]

Please review this change to add release barriers when allocating
objects that may be visible to a concurrent collector.

This change only addresses the allocation side, where the
initialization occurs; other subtasks of JDK-8160369 will address the
(mostly GC-specific) readers that need to accomodate in-progress
allocations.

As part of this change, eliminated post_allocation_install_obj_klass,
which consisted of a call to [release_]set_klass + assertions that are
redundant with those in that called function.

CR:
https://bugs.openjdk.java.net/browse/JDK-8165808

Webrev:
http://cr.openjdk.java.net/~kbarrett/8165808/webrev.00/

Testing:
Local jtreg tests.