RFR: 8247281: migrate ObjectMonitor::_object to OopStorage

[Daniel D.Daugherty]

This RFE is to migrate the following field to OopStorage:

class ObjectMonitor {
<snip>
  void* volatile _object; // backward object pointer - strong root

Unlike the previous patches in this series, there are a lot of collateral
changes so this is not a trivial review. Sorry for the tedious parts of
the review. Since Erik and I are both contributors to this patch, we
would like at least 1 GC team reviewer and 1 Runtime team reviewer.

This changeset was tested with Mach5 Tier[1-3],4,5,6,7,8 testing
along with JDK-8252980 and JDK-8252981. I also ran it through my
inflation stress kit for 48 hours on my Linux-X64 machine.

-------------

Commit messages:
 - 8247281: migrate ObjectMonitor::_object to OopStorage

Changes: https://git.openjdk.java.net/jdk/pull/135/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=135&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8247281
  Stats: 445 lines in 36 files changed: 108 ins; 234 del; 103 mod
  Patch: https://git.openjdk.java.net/jdk/pull/135.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/135/head:pull/135

PR: https://git.openjdk.java.net/jdk/pull/135