RFR: 8256497: Zero: enable G1 and Shenandoah GCs

[Erik Joelsson]

On Tue, 17 Nov 2020 19:02:03 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Following the [JDK-8255796](https://bugs.openjdk.java.net/browse/JDK-8255796) improvement that ditched the inline contiguous alloc use from Zero, we can now rely on GC interface to hook the GCs properly. G1 and Shenandoah are a bit special here, because they require special `Reference.get` handling.
> 
> Note that it does not change the default GC for Zero, because Zero is implicitly `NeverActAsServerMachine`, which still selects Serial GC by default. After this change, Zero users can opt-in to G1 or Shenandoah.
> 
> Additional testing:
>  - [x] Linux x86_64 Zero fastdebug `hotspot_gc_shenandoah` (some lingering failures about non-enabled compressed oops)
>  - [ ] Linux x86_64 Zero fastdebug `tier1` with `-XX:+UseG1GC`

Build change looks ok.

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Marked as reviewed by erikj (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/1268