RFR: Enable 1-step workflow with Shenandoah GC [v3]

[Aleksey Shipilev]

> WIP, includes upstream [JDK-8293650](https://bugs.openjdk.org/browse/JDK-8293650).
> 
> I checked this works well:
> 
> 
> #!/bin/bash
> make images
> J=build/macosx-aarch64-server-fastdebug/images/jdk/bin/java
> rm -fv JavacBenchApp.cds*
> $J -XX:+UseShenandoahGC -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
> $J -XX:+UseShenandoahGC -XX:CacheDataStore=JavacBenchApp.cds -cp JavacBenchApp.jar JavacBenchApp 50
> 
> 
> I'll look around what other tests I need to run.

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains two commits:

 - Make sure LeydenGCFlags passes when some GCs are not configured
 - Shenandoah support

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Changes: https://git.openjdk.org/leyden/pull/8/files
  Webrev: https://webrevs.openjdk.org/?repo=leyden&pr=8&range=02
  Stats: 73 lines in 6 files changed: 49 ins; 3 del; 21 mod
  Patch: https://git.openjdk.org/leyden/pull/8.diff
  Fetch: git fetch https://git.openjdk.org/leyden.git pull/8/head:pull/8

PR: https://git.openjdk.org/leyden/pull/8