RFR: 8256011: Shenandoah: Don't resurrect finalizably reachable objects [v6]

[Roman Kennke]

> In the weak-LRB we currently return referents when it is 'marked', that is when it's either reachable strongly or through a finalizable object. This means a finalizable object can be resurrected by Reference.get(), which is wrong. Only truly strongly reachable objects should be returned by Reference.get() during weak-reference-processing. 
> 
> I had to reconsider the way we call into runtime-LRBs from generated code for these reasons:
>  - We need to distinguish phantom, weak and strong reference strength, and native vs in-heap access. Those are two orthogonal dimensions
>  - We can have strong and phantom native referents, and strong and weak in-heap referents
>  - Native referents are never compressed
> 
> Note that this depends on PR#1140.
> 
> Testing:
>  - [x] hotspot_gc_shenandoah (x86_64, x64_32, aarch64)
>  - [x] tier1 +UseShenandoahGC +ShenandoahVerify
>  - [ ] tier2 +UseShenandoahGC +ShenandoahVerify

Roman Kennke has updated the pull request incrementally with two additional commits since the last revision:

 - Remove superfluous LP64 in aarch64 part
 - Fixes/missing parts for x86_64

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/1109/files
  - new: https://git.openjdk.java.net/jdk/pull/1109/files/d361e60b..37c97786

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=1109&range=05
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=1109&range=04-05

  Stats: 20 lines in 3 files changed: 4 ins; 6 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1109.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1109/head:pull/1109

PR: https://git.openjdk.java.net/jdk/pull/1109