8318422: Allow poller threads be virtual threads

[shipilev]

Fixes a Loom performance problem.

Additional testing:

• Linux x86_64 fastdebug, all passes
• Linux x86_64 fastdebug, all passes with Virtual scheduler

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Progress

• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8318422 needs maintainer approval

Issue

• JDK-8318422: Allow poller threads be virtual threads (Enhancement - P3)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk21u-dev.git pull/247/head:pull/247
$ git checkout pull/247

Update a local copy of the PR:
$ git checkout pull/247
$ git pull https://git.openjdk.org/jdk21u-dev.git pull/247/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 247

View PR using the GUI difftool:
$ git pr show -t 247

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk21u-dev/pull/247.diff

[bridgekeeper]

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[openjdk]

This backport pull request has now been updated with issue from the original commit.

[openjdk]

❗ This change is not yet ready to be integrated.
See the Progress checklist in the description for automated requirements.

[bridgekeeper]

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