8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation

[tabata-d]

This fix addresses a usability and consistency issue in the HotSpot VM by renaming the command-line flag DisableTHPStackMitigation to THPStackMitigation. The original flag's name created a confusing double negation, where enabling the flag (+DisableTHPStackMitigation) actually disabled the underlying mitigation. The renaming resolves this by adopting a direct naming convention, where -XX:+THPStackMitigation enables the mitigation and -XX:-THPStackMitigation disables it, aligning with standard JDK flag practices.

We are backporting this fix from JDK 17 to JDK 11. This is considered an unclean backport due to minor Git diff differences from previous versions, such as JDK 17 using product where JDK 11 used diagnostic, and differing FLAG_SET_ERGO macro arguments.

Testing:
A full Hotspot jtreg run was executed on a Linux x86_64 machine and all are passed

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Progress

• Change must be properly reviewed (1 review required, with at least 1 Reviewer)
• Change must not contain extraneous whitespace
• Commit message must refer to an issue
• JDK-8312585 needs maintainer approval

Integration blocker

 ⚠️ Dependency #3210 must be integrated first

Issue

• JDK-8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation (Enhancement - P4)

Reviewing

Using git

Checkout this PR locally:
$ git fetch https://git.openjdk.org/jdk11u-dev.git pull/3211/head:pull/3211
$ git checkout pull/3211

Update a local copy of the PR:
$ git checkout pull/3211
$ git pull https://git.openjdk.org/jdk11u-dev.git pull/3211/head

Using Skara CLI tools

Checkout this PR locally:
$ git pr checkout 3211

View PR using the GUI difftool:
$ git pr show -t 3211

Using diff file

Download this PR as a diff file:
https://git.openjdk.org/jdk11u-dev/pull/3211.diff

Using Webrev

Link to Webrev Comment

[bridgekeeper]

👋 Welcome back dtabata! A progress list of the required criteria for merging this PR into pr/3210 will be added to the body of your pull request. There are additional pull request commands available for use with this pull request.

[openjdk]

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

[openjdk]

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

[mlbridge]

Webrevs

• 01: Full - Incremental (e542239a)
• 00: Full (e542239a)

[openjdk-notifier]

The parent pull request that this pull request depends on has been closed without being integrated and the target branch of this pull request has been updated as the previous branch was deleted. This means that changes from the parent pull request will start to show up in this pull request. If closing the parent pull request was done in error, it will need to be re-opened and this pull request will need to manually be retargeted again.

[tabata-d]

After the discussion in #3210, I have decided to close this PR.