RFR: 8341913: Support CDS heap dumping for Shenandoah and Epsilon [v2]

[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`

Aleksey Shipilev has updated the pull request incrementally with one additional commit since the last revision:

  Wrap test with Shenandoah check

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/21457/files
  - new: https://git.openjdk.org/jdk/pull/21457/files/69412929..0b4205ed

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=21457&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=21457&range=00-01

  Stats: 3 lines in 1 file changed: 2 ins; 0 del; 1 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