RFR: 8298059: Linked stack watermarks don't support nesting [v3]

[Erik Österlund]

> The KeepStackGCProcessedMark facility used to ensure a thread (possibly remote) remains fully processed in its scope, has not yet had to support nesting. As it turns out, they can now nest after JDK-8294924. So this patch adds support for nesting. Tested ZGC specific tests tier 1-8, general testing 1-3 and manually tested the two tests that failed and got problem listed.

Erik Österlund has updated the pull request incrementally with one additional commit since the last revision:

  Change from mtInternal to mtThread

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Changes:
  - all: https://git.openjdk.org/jdk/pull/11530/files
  - new: https://git.openjdk.org/jdk/pull/11530/files/023b752b..39a4eea0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=11530&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=11530&range=01-02

  Stats: 4 lines in 2 files changed: 1 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/11530.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11530/head:pull/11530

PR: https://git.openjdk.org/jdk/pull/11530