RFR: 8286360: ARM32: Fix crashes after JDK-8284161 (Virtual Threads) [v2]

[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

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 10 additional commits since the last revision:

 - Merge branch 'master' into JDK-8286360-loom-arm32-no-preview
 - 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:
  - all: https://git.openjdk.java.net/jdk/pull/8634/files
  - new: https://git.openjdk.java.net/jdk/pull/8634/files/dff69a64..0eb6f85f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8634&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8634&range=00-01

  Stats: 10069 lines in 203 files changed: 8448 ins; 1137 del; 484 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