RFR: 8338912: CDS: Segmented roots array

[Aleksey Shipilev]

Attempt to drop the min region alignment with [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828) highlights an interesting trouble. The roots array we are creating during the dump time can easily be larger than the min region alignment. We are currently "lucky" none of our tests hit this limit. AFAICS, about 128K classes would be enough to hit the current 1M min region alignment. Dropping the min region alignment to 256K starts to fail the test with "only" 30K classes, [JDK-8338856](https://bugs.openjdk.org/browse/JDK-8338856).

We can slice that heap root array, and thus untie the roots count from the min region alignment. I am submitting something that works, but this might not be the final form for it. I would like @iklam to poke holes in this approach :)

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `runtime/cds`
 - [ ] linux-aarch64-server-fastdebug, `all`
 - [ ] linux-x86_64-server-fastdebug, `all`

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Commit messages:
 - Segmented heap roots

Changes: https://git.openjdk.org/jdk/pull/20858/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20858&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338912
  Stats: 233 lines in 10 files changed: 133 ins; 46 del; 54 mod
  Patch: https://git.openjdk.org/jdk/pull/20858.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20858/head:pull/20858

PR: https://git.openjdk.org/jdk/pull/20858