RFR: 8271121: ZGC: stack overflow (segv) when -Xlog:gc+start=debug

[Per Liden]

When an allocation stall happens we're trying to access oops (the thread and thread name) while logging. We shouldn't do that since it can lead to a recursive allocation stall situation, which eventually will cause a stack overflow. I suggest we simply don't log anything in ZStatCriticalPhase::register_start(). The useful part of the logging happens in ZStatCriticalPhase::register_end() anyway, where it's safe to access oops.

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Commit messages:
 - 8271121: ZGC: stack overflow (segv) when -Xlog:gc+start=debug

Changes: https://git.openjdk.java.net/jdk/pull/4990/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4990&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8271121
  Stats: 4 lines in 1 file changed: 2 ins; 0 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4990.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4990/head:pull/4990

PR: https://git.openjdk.java.net/jdk/pull/4990