RFR: Loom: PPC64, S390, ARM32, Zero stubs

[Aleksey Shipilev]

On Tue, 5 Oct 2021 16:44:01 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> This WIP provides enough stubs to make Loom cross-compilable to PPC64, S390, ARM32 and Zero. This should be enough to get the sense where the shared code actually platform-specific (referencing stuff from `frame` that is platform-specific), what should be implemented for a platform, and also should be enough to get GHA build checks green. Note current patch does not implement Loom on these platforms, but merely provides the absolute basic build capabilities.
> 
> Note that ARM32 builds would not be clean until #65 is integrated with 32-bit cleanliness fixes.
> 
> Additional testing:
>  - [x] Linux x86_64 tier1_loom still passes
>  - [x] Linux PPC64 cross-compilation
>  - [x] Linux S390 cross-compilation
>  - [x] Linux ARM32 cross-compilation
>  - [x] Linux x86_64 Zero (Hotspot) compilation

Any comments? I would like to have it integrated.

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PR: https://git.openjdk.java.net/loom/pull/72