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

[Aleksey Shipilev]

> 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

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

Changes:
  - all: https://git.openjdk.java.net/loom/pull/72/files
  - new: https://git.openjdk.java.net/loom/pull/72/files/7195f357..a0906d48

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=loom&pr=72&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=loom&pr=72&range=00-01

  Stats: 672 lines in 28 files changed: 110 ins; 473 del; 89 mod
  Patch: https://git.openjdk.java.net/loom/pull/72.diff
  Fetch: git fetch https://git.openjdk.java.net/loom pull/72/head:pull/72

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