RFR: 8253183: Fragile memory barrier selection for some weak memory model platforms

[Martin Doerr]

Hi, I'd like to propose a fix after the discussion in https://bugs.openjdk.java.net/browse/JDK-8253183

Please especially review my new comments in the code. The authors of the surrounding code in objectMonitor.cpp and
synchronizer.cpp may know better.

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Commit messages:
 - 8253183: Fragile memory barrier selection for some weak memory model platforms

Changes: https://git.openjdk.java.net/jdk/pull/387/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=387&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253183
  Stats: 32 lines in 4 files changed: 11 ins; 9 del; 12 mod
  Patch: https://git.openjdk.java.net/jdk/pull/387.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/387/head:pull/387

PR: https://git.openjdk.java.net/jdk/pull/387