RFR: 8257668: SA JMap - skip non-java thread stack dump for heap dump

[Lin Zang]

when use SA JMap dump commands, such as "jhsdb jmap --binaryheap" or "dump heap" with "jhsdb clhsdb", it keep printing "dumpStack: not java Thread.". 
Skip non-java thread stack dump to avoid printing the message.

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Commit messages:
 - 8257668: SA JMap - skip non-java thread stack dump for heap dump

Changes: https://git.openjdk.java.net/jdk/pull/1593/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=1593&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8257668
  Stats: 4 lines in 1 file changed: 3 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/jdk/pull/1593.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/1593/head:pull/1593

PR: https://git.openjdk.java.net/jdk/pull/1593