RFR: 8286476: x86_32: Fix crashes with non-preview mode after JDK-8284161 (Virtual Threads) [v4]

[Aleksey Shipilev]

> Virtual Threads are not fully implemented on x86_32. The feature is guarded with `--enable-preview`. But there is some VM code that is executed unconditionally anyway, crashing the VM on some tests. This improves the x86_32 code to the point that tests without `--enable-preview` run successfully. This adds the scaffolding for full implementation too. 
> 
> Additional testing:
>  - [x] Linux x86_32 fastdebug, `tier1` failures only with `--enable-preview`
>  - [x] Linux x86_32 fastdebug, `tier2` failures only with `--enable-preview`

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 six additional commits since the last revision:

 - Rebasing after JDK-8286480
 - Merge branch 'master' into JDK-8286476-loom-x86-32-no-preview
 - x86-64 code still a bit wrong
 - Fix x86_64 release build
 - Fallback for 32-bit port signature helpers
 - Fix jdk_jfr

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8630/files
  - new: https://git.openjdk.java.net/jdk/pull/8630/files/5dddb9ba..3d510b80

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8630&range=03
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8630&range=02-03

  Stats: 470 lines in 34 files changed: 201 ins; 199 del; 70 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8630.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8630/head:pull/8630

PR: https://git.openjdk.java.net/jdk/pull/8630