RFR: 8293650: Shenandoah: Support archived heap objects [v5]

[Aleksey Shipilev]

> This implements CDS Java heap loading for Shenandoah. There are peculiarities with how CDS loads objects: it basically asks for a contiguous block of memory, fills it out, potentially relocating the objects. This gets interesting when a single Shenandoah region cannot contain the entirety of the load. See the implementation for gory details.
> 
> Current implementation would work well only with Shenandoah heap regions >= 1M, in other words, with the heaps >=2G. It would be better if we trim down the min alignment, thus unblocking smaller heaps. It is not necessary to do so in this PR,  so I track that work separately: [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828).
> 
> Additional testing:
>  - [x] New test
>  - [x] Linux AArch64 server fastdebug, `all` with `-XX:+UseShenandoahGC -XX:+ShenandoahVerify`
>  - [x] Same as above, but `MIN_GC_REGION_ALIGNMENT` manually dropped to 256K (mimics [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828))

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Wrap the whole thing in CDS define

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/20468/files
  - new: https://git.openjdk.org/jdk/pull/20468/files/7486a1e7..d8984aa4

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20468&range=04
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20468&range=03-04

  Stats: 13 lines in 1 file changed: 5 ins; 7 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/20468.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20468/head:pull/20468

PR: https://git.openjdk.org/jdk/pull/20468