RFR: 8314091: Set native thread name for Java main thread

[Yi Yang]

`cat /proc/<java_process>/task/<main thread>/comm` shows "java"

User wants it to be "main", just like any other JVM threads, which can be achieved by explicitly calling pthread_setname_np when spawning main thread at VM startup.

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Commit messages:
 - 8314091: Set native thread name for Java main thread

Changes: https://git.openjdk.org/jdk/pull/15224/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15224&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314091
  Stats: 7 lines in 1 file changed: 6 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/15224.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15224/head:pull/15224

PR: https://git.openjdk.org/jdk/pull/15224