[lworld] RFR: 8369134: [lworld] We should preserve the markWord for all GCs [v2]

[Paul Hübner]

> Hi all,
> 
> This PR follows up on changes made in [JDK-8368875](https://bugs.openjdk.org/browse/JDK-8368875). I updated the documentation to reflect the new information gained.
> 
> Furthermore, the investigation described in the issue yielded that we _must_ preserve the larval bit explicitly. I introduce a regression test. If we change the mark word preservation condition to `return (!is_unlocked() || !has_no_hash())` (i.e. what we had before), I have confirmed that we will crash a debug VM via failed assertion. Note that ZGC does not require mark word preservation. I've nonetheless decided to include it in case this behaviour is introduced later.
> 
> Testing: tiers 1-3.

Paul Hübner has updated the pull request incrementally with one additional commit since the last revision:

  WhiteBox API.

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Changes:
  - all: https://git.openjdk.org/valhalla/pull/1666/files
  - new: https://git.openjdk.org/valhalla/pull/1666/files/cd2c092c..a75ca117

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=valhalla&pr=1666&range=01
 - incr: https://webrevs.openjdk.org/?repo=valhalla&pr=1666&range=00-01

  Stats: 55 lines in 1 file changed: 30 ins; 17 del; 8 mod
  Patch: https://git.openjdk.org/valhalla/pull/1666.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1666/head:pull/1666

PR: https://git.openjdk.org/valhalla/pull/1666