RFR: 8341913: Support CDS heap dumping for Shenandoah and Epsilon

[Aleksey Shipilev]

This follows [JDK-8298614](https://bugs.openjdk.org/browse/JDK-8298614) and completes support for CDS dumping for Shenandoah and Epsilon. We have already enabled these in Leyden repo. I took a chance to clean up the code a bit. Shenandoah support is tested by the existing test. Epsilon support was tested manually, since it is quite likely flaky (= depends on pre-dump conditions not to run into OOM) to include in tests.

Additional testing:
 - [x] Linux x86_64 fastdebug server, `runtime/cds`
 - [x] Linux x86_64 fastdebug server, `runtime/cds`, `-XX:+UseShenandoahGC`
 - [x] Linux x86_64 fastdebug server, manual dumps with `-XX:+UseEpsilonGC`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21457/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21457&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341913
  Stats: 9 lines in 2 files changed: 6 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/21457.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21457/head:pull/21457

PR: https://git.openjdk.org/jdk/pull/21457