RFR: 8266742: Check W^X state on possible safepoint [v2]

[Anton Kozlov]

> Hi,
> 
> Please review a check for W^X mode at safepoint. As described in the bug, this explicit check aims to catch the wrong W^X mode that may lead to a crash (such crashes are rather intermittent).
> 
> This check helped to verify the fix for https://bugs.openjdk.java.net/browse/JDK-8265292 and should help to discover similar issues. It is disabled by default to avoid unexpected failures in the regular use, but eventually it should be turned on unconditionally.
> 
> I still owe a complete W^X approach description. Hope this patch does not depend much on that.

Anton Kozlov has updated the pull request incrementally with one additional commit since the last revision:

  Change option and method name

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Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3920/files
  - new: https://git.openjdk.java.net/jdk/pull/3920/files/f979fd08..589db510

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3920&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3920&range=00-01

  Stats: 6 lines in 3 files changed: 1 ins; 0 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3920.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3920/head:pull/3920

PR: https://git.openjdk.java.net/jdk/pull/3920