RFR: 8295214: Generational ZGC: Guard nmethods from cross modifying code

[Erik Österlund]

Generational ZGC will need to patch nmethod instructions outside of safepoints, and guard entries into the nmethods with cross modifying code fences. This is mostly taken care of by nmethod entry barrier code. But there are a few entries that don't go through nmethod entry barriers that need fixing. In particular when entering an nmethod by returning through the stack watermark barrier. This patch ensures that whenever the stack watermark barrier exposes a new nmethod, we also ensure that a cross modify fence is executed, so that any concurrently updated instructions can be safely executed.

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Commit messages:
 - 8295214: Generational ZGC: Guard nmethods from cross modifying code

Changes: https://git.openjdk.org/jdk/pull/11042/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11042&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8295214
  Stats: 25 lines in 3 files changed: 20 ins; 5 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/11042.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11042/head:pull/11042

PR: https://git.openjdk.org/jdk/pull/11042