RFR: 8268857: Merge three vm operations in thread_dump

[Denghui Dong]

Hi,

Could I have a review of this change that merges three vm operations(VM_PrintThreads, VM_PrintJNI, VM_FindDeadlocks) in thread_dump and signal_thread_entry.

`jstack` is a very common command, even in the production environment.

In addition to reduce the cost of entering safepoint, I think this patch also could ensure the consistency of the results of VM_PrintThreads and VM_FindDeadlocks.

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Commit messages:
 - 8268857: Merge three vm operations in thread_dump

Changes: https://git.openjdk.java.net/jdk/pull/4504/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4504&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8268857
  Stats: 38 lines in 5 files changed: 24 ins; 10 del; 4 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4504.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4504/head:pull/4504

PR: https://git.openjdk.java.net/jdk/pull/4504