RFR: Loom: PPC64, S390, ARM32, Zero stubs [v2]

[Ron Pressler]

On Mon, 11 Oct 2021 16:34:54 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
>
> Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:
> 
>  - Merge branch 'fibers' into platform-stubs
>  - More whitespace
>  - Self-review fixes
>  - PPC64, S390, ARM32, Zero stubs

Marked as reviewed by rpressler (Lead).

Good to go. Thank you!

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