RFR: 8278239: vmTestbase/nsk/jvmti/RedefineClasses/StressRedefine failed with EXCEPTION_ACCESS_VIOLATION at 0x000000000000000d

[Coleen Phillimore]

Thanks to @stefank and @fisk for the diagnosis.  ZGC is looking at metadata in unloaded nmethods, but redefinition doesn't keep this metadata from being deallocated.  Change the iterators that mark metadata in use to walk unloaded nmethods as well as other alive nmethods.

The test case reproduces the crash on windows if run in an 100 iteration loop.  This fix does not crash in the same test.  Also ran tier1-6.

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Commit messages:
 - 8278239: vmTestbase/nsk/jvmti/RedefineClasses/StressRedefine failed with EXCEPTION_ACCESS_VIOLATION at 0x000000000000000d

Changes: https://git.openjdk.java.net/jdk/pull/6900/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6900&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278239
  Stats: 4 lines in 1 file changed: 0 ins; 0 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6900.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6900/head:pull/6900

PR: https://git.openjdk.java.net/jdk/pull/6900