[master] RFR: OMWorld: Cleanup ObjectMonitor::_header [v4]

[Axel Boldt-Christmas]

> This changes the `ObjectMonitor::_header` to `ObjectMonitor::_metadata` and cleans up the surrounding terminology. 
> 
> Parts of this could be upstreamed, but the distinction of this field between the different locking modes is introduced in OMWorld. 
> 
> This patch will conflict with Lilliput if rebased on-top of OMWorld because Lilliput introduces `static ByteSize header_offset()` which is not used after OMWorld.  They could probably be removed in both these patches.
> 
> This also changes all the fields to have the same access level (private) so that this can be a standard layout type.

Axel Boldt-Christmas has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains four commits:

 - Merge remote-tracking branch 'upstream_lilliput/master' into lilliput-om-cleanup-objectmonitor-header
 - Use variable directly in sizeof
 - Update src/hotspot/share/runtime/objectMonitor.hpp
   
   Co-authored-by: Stefan Karlsson <stefan.karlsson at oracle.com>
 - Cleanup ObjectMonitor::_header

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Changes: https://git.openjdk.org/lilliput/pull/161/files
  Webrev: https://webrevs.openjdk.org/?repo=lilliput&pr=161&range=03
  Stats: 94 lines in 8 files changed: 21 ins; 10 del; 63 mod
  Patch: https://git.openjdk.org/lilliput/pull/161.diff
  Fetch: git fetch https://git.openjdk.org/lilliput.git pull/161/head:pull/161

PR: https://git.openjdk.org/lilliput/pull/161