RFR: 8312585: Rename DisableTHPStackMitigation flag to THPStackMitigation

[Aleksey Shipilev]

The flag newly added by [JDK-8312182](https://bugs.openjdk.org/browse/JDK-8312182) is prompting the use of `-XX:+DisableTHPStackMitigation` to disable the THP stack mitigation, thus allowing THP in thread stacks. This double negation does not read well, and not in line with other mitigation flags like `IntelJccErratumMitigation`.

It would be better to rename the flag to avoid double-negation, before it proliferates to other JDK releases.

(I would have the same comment during the original review, but missed it :P)

Additional testing: 
 - [ ] GHA
 - [ ] Linux x86_64 fastdebug, affected test

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/14992/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14992&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8312585
  Stats: 12 lines in 3 files changed: 2 ins; 2 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/14992.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14992/head:pull/14992

PR: https://git.openjdk.org/jdk/pull/14992