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

[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

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Commit messages:
 - More whitespace
 - Self-review fixes
 - PPC64, S390, ARM32, Zero stubs

Changes: https://git.openjdk.java.net/loom/pull/72/files
 Webrev: https://webrevs.openjdk.java.net/?repo=loom&pr=72&range=00
  Stats: 2286 lines in 49 files changed: 2203 ins; 33 del; 50 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