RFR: 8340166: [REDO] CDS: Trim down minimum GC region alignment

[Aleksey Shipilev]

Now that [JDK-8338912](https://bugs.openjdk.org/browse/JDK-8338912) is in, we can try to redo [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828). See the rationale for this change in [JDK-8337828](https://bugs.openjdk.org/browse/JDK-8337828) PR: https://github.com/openjdk/jdk/pull/20469. This change is identical to that PR.

Additional testing:
 - [x] Linux x86_64 server fastdebug, `runtime/cds`
 - [x] Linux x86_64 server fastdebug, `runtime/cds` `-XX:-UseCompressedOops` (failed during the original attempt to do this)
 - [x] Linux x86_64 server fastdebug, `runtime/cds` `-XX:+UseShenandoahGC`
 - [x] Linux x86_64 server fastdebug, `runtime/cds` `-XX:+UseShenandoahGC -XX:-UseCompressedOops`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21011/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21011&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8340166
  Stats: 5 lines in 1 file changed: 0 ins; 1 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/21011.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21011/head:pull/21011

PR: https://git.openjdk.org/jdk/pull/21011