RFR: 8280844: Epoch shift synchronization point for Compiler threads is inadequate

[Markus Grönlund]

Greetings,

[JDK-8233111](https://bugs.openjdk.java.net/browse/JDK-8233111) attempted to address artefact tagging for Compiler threads, letting threads run _thread_in_native to avoid the transition. Unfortunately, that attempt proved inadequate.

The epoch race is avoided only by performing the transition to _thread_in_vm.

Testing: jdk_jfr

Thanks
Markus

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Commit messages:
 - 8280844

Changes: https://git.openjdk.java.net/jdk/pull/8724/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8724&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8280844
  Stats: 102 lines in 6 files changed: 14 ins; 84 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8724.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8724/head:pull/8724

PR: https://git.openjdk.java.net/jdk/pull/8724