RFR: 8247281: migrate ObjectMonitor::_object to OopStorage [v4]

[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.

Daniel D. Daugherty has updated the pull request incrementally with one additional commit since the last revision:

  kimbarrett CR - made minor changes to address Kim's code review.

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/135/files
  - new: https://git.openjdk.java.net/jdk/pull/135/files/bbf8dbd0..9fa2bed1

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=135&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=135&range=02-03

  Stats: 16 lines in 2 files changed: 11 ins; 3 del; 2 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