RFR: 8305898: Alternative self-forwarding mechanism [v17]

[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 incrementally with two additional commits since the last revision:

 - Merge branch 'JDK-8305896' into JDK-8305898
 - Remove G1-only assert for fallback forwarding, and comment with explanation

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13779/files
  - new: https://git.openjdk.org/jdk/pull/13779/files/4895ad86..3519da72

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13779&range=16
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13779&range=15-16

  Stats: 5 lines in 1 file changed: 4 ins; 0 del; 1 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