RFR: 8268857: Merge three vm operations in thread_dump [v3]

[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.

Denghui Dong has updated the pull request incrementally with two additional commits since the last revision:

 - add cutoff support for find_deadlocks_at_safepoint
 - remove useless field '_is_deadlock'

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4504/files
  - new: https://git.openjdk.java.net/jdk/pull/4504/files/56cd2a8b..42faa20b

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4504&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4504&range=01-02

  Stats: 53 lines in 4 files changed: 36 ins; 8 del; 9 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