RFR: 8341091: CDS: Segmented roots array misses roots

[Aleksey Shipilev]

There are a few bugs in [JDK-8338912](https://bugs.openjdk.org/browse/JDK-8338912) that we discovered in Leyden that uses multiple segments: a) root_index in ArchiveHeapWriter::copy_roots_to_buffer is initialized to zero for every segment, so all segments have the same set of roots; b) HeapShared::get_root and friends compute segment and internal indices modulo _byte_ size, while they should instead do it modulo _element_ size.

This does not affect current mainline directly, since we run with a single segment the overwhelming majority of the time.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `runtime/cds`
 - [x] Linux x86_64 server fastdebug, `runtime/cds` + `-XX:-UseCompressedOops`
 - [x] Transplanted the patch to Leyden repo and verified it passes `runtime/cds`
 - [ ]  Linux x86_64 server fastdebug, `all`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21221/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21221&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341091
  Stats: 43 lines in 5 files changed: 21 ins; 11 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/21221.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21221/head:pull/21221

PR: https://git.openjdk.org/jdk/pull/21221