RFR: 8293650: Shenandoah: Support archived heap objects

[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
 - [ ] Linux AArch64 server fastdebug, `all` with `-XX:+UseShenandoahGC -XX:+ShenandoahVerify`
 - [ ] Same as above, but `MIN_GC_REGION_ALIGNMENT` manually dropped to 256K

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Commit messages:
 - Touchups in test
 - Basic implementation, works well, passes tests

Changes: https://git.openjdk.org/jdk/pull/20468/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8293650
  Stats: 193 lines in 7 files changed: 183 ins; 6 del; 4 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