RFR: 8305898: Alternative self-forwarding mechanism [v4]

[Roman Kennke]

> Currently, the Serial, Parallel and G1 GCs store a pointer to self into object headers, when compaction fails, to indicate that the object has been looked at, but failed compaction into to-space. This is problematic for compact object headers ([JDK-8294992](https://bugs.openjdk.org/browse/JDK-8294992)) because it would (temporarily) over-write the crucial class information, which we need for heap parsing. I would like to propose an alternative: use the bit #3 (previously biased-locking bit) to indicate that an object is 'self-forwarded'. That preserves the crucial class information in the upper bits of the header until the full header gets restored.

Roman Kennke has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains ten commits:

 - Merge branch 'JDK-8305896' into JDK-8305898
 - Merge branch 'JDK-8305896' into JDK-8305898
 - Merge branch 'JDK-8305896' into JDK-8305898
 - Merge branch 'JDK-8305896' into JDK-8305898
 - Use forwardee() in forward_to_atomic() method
 - Merge branch 'JDK-8305896' into JDK-8305898
 - Merge branch 'JDK-8305896' into JDK-8305898
 - Replace uses of decode_pointer() with forwardee()
 - 8305898: Alternative self-forwarding mechanism

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Changes: https://git.openjdk.org/jdk/pull/13779/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=13779&range=03
  Stats: 85 lines in 8 files changed: 69 ins; 2 del; 14 mod
  Patch: https://git.openjdk.org/jdk/pull/13779.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13779/head:pull/13779

PR: https://git.openjdk.org/jdk/pull/13779