RFR: 8282721: HotSpot Style Guide should allow considered use of C++ thread_local

[David Holmes]

Style guide changes to support JDK-8282469 (PR https://github.com/openjdk/jdk/pull/7719). We no longer prohibit use of C++ `thread_local`, but allow it when there is an essential, and considered, need.

This is a modification of the Style Guide, so rough consensus among the HotSpot Group members is required to make this change. Only Group members should vote for approval (via the github PR), though reasoned objections or comments from anyone will be considered. A decision on this proposal will not be made before Friday 18-Mar-2022 at 12h00 UTC.

Since we're piggybacking on github PRs here, please use the PR review process to approve (click on Review Changes > Approve), rather than sending a "vote: yes" email reply that would be normal for a CFV.

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Commit messages:
 - 8282721: HotSpot Style Guide should allow considered use of C++ thread_local

Changes: https://git.openjdk.java.net/jdk/pull/7720/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=7720&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8282721
  Stats: 18 lines in 2 files changed: 5 ins; 0 del; 13 mod
  Patch: https://git.openjdk.java.net/jdk/pull/7720.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/7720/head:pull/7720

PR: https://git.openjdk.java.net/jdk/pull/7720