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

[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`

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Commit messages:
 - Fallback for 32-bit port signature helpers
 - Fix jdk_jfr

Changes: https://git.openjdk.java.net/jdk/pull/8630/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8630&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286476
  Stats: 145 lines in 2 files changed: 135 ins; 6 del; 4 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