RFR: 8286360: ARM32: Fix crashes after JDK-8284161 (Virtual Threads)

[Aleksey Shipilev]

Virtual Threads are not fully implemented on ARM32, but there is a fallback implementation that can be used. Still, there is some VM code that is executed unconditionally anyway, crashing the VM on some tests. This improves the ARM32 code to the point that VM is able to build itself and pass tests. This adds the scaffolding for full implementation too.

Additional testing:
 - [x] Linux arm32 fastdebug build
 - [x] Linux arm32 fastdebug, `jdk_loom  hotspot_loom` pass

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Commit messages:
 - Whitespace and comments
 - JFR barriers from Sergey
 - Cleanups
 - Flesh out generate_jfr_write_checkpoint a bit (untested)
 - Match the generator shape to x86_64
 - C2 patch from Sergey
 - Merge branch 'master' into JDK-8286360-loom-arm32-no-preview
 - Merge branch 'master' into JDK-8286360-loom-arm32-no-preview
 - ARM32 bootstraps fine without C2

Changes: https://git.openjdk.java.net/jdk/pull/8634/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8634&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286360
  Stats: 201 lines in 7 files changed: 140 ins; 38 del; 23 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8634.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8634/head:pull/8634

PR: https://git.openjdk.java.net/jdk/pull/8634