RFR: 8305898: Alternative self-forwarding mechanism

[Roman Kennke]

Currently, the Serial, Parallel and G1 GCs store a pointer to self into object headers, when promotion fails, to indicate that the object has been looked at, but failed promotion. 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.

A side-effect of this is that we can get rid of the machinery to preserve headers across promotion failures in Serial and G1. (Parallel GC [ab]uses the preserved-headers structure to also find all the forwarded objects for header restoration. This could be changed, I suppose, but it is not trivial.) If you prefer, I could break-out the removal of the preserved-headers stuff into a separate PR.

This is in preparation of upstreaming compact object headers, and I intend to push it only once all the parts have been approved.

Testing:
 - [x] tier1
 - [x] tier2
 - [x] tier3
 - [x] tier4
 - [x] Running in production @ AWS since >1year without troubles

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Commit messages:
 - 8305898: Alternative self-forwarding mechanism

Changes: https://git.openjdk.org/jdk/pull/20603/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20603&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8305898
  Stats: 156 lines in 18 files changed: 66 ins; 66 del; 24 mod
  Patch: https://git.openjdk.org/jdk/pull/20603.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20603/head:pull/20603

PR: https://git.openjdk.org/jdk/pull/20603