RFR: 8286476: x86_32: Fix crashes with non-preview mode after JDK-8284161 (Virtual Threads) [v2]

[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 incrementally with one additional commit since the last revision:

  Fix x86_64 release build

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/8630/files
  - new: https://git.openjdk.java.net/jdk/pull/8630/files/e1f7a5f2..621424b2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=8630&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=8630&range=00-01

  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 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