RFR: 8298377: JfrVframeStream causes deadlocks in ZGC

[Erik Österlund]

The JfrVFrameStream is used while generating stack traces for events. One of the events are the ZPage allocation event. This event is sometimes sent when ZGC is relocating. The current implementation of JfrVFrameStream uses WalkContinuation::include, which causes JFR to walk the continuation and perform GC barriers. This is problematic, since ZGC has a requirement that we never perform load barriers while running the relocation code. If we do, we might end up performing other reloctions from the the relocation code, and in some cases that causes dead locks. 

I propose that JFR doesn't walk the stack during stack watermark processing. This PR implements that change. This PR replaces https://github.com/openjdk/jdk/pull/11586

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Commit messages:
 - 8298377: JfrVframeStream causes deadlocks in ZGC

Changes: https://git.openjdk.org/jdk/pull/11778/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11778&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8298377
  Stats: 189 lines in 10 files changed: 160 ins; 12 del; 17 mod
  Patch: https://git.openjdk.org/jdk/pull/11778.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11778/head:pull/11778

PR: https://git.openjdk.org/jdk/pull/11778