RFR: JDK-8322237: Heap dump contains duplicate thread records for mounted virtual threads

[Alex Menkov]

HeapDumper dumps virtual threads in 2 places:
- dumping platform threads (mounted virtual threads are dumped as separate thread object);
- dumping heap objects when the object is `java.lang.VirtualThread`.

In the 2nd case mounted virtual threads should be skipped (as they are already dumped with correct stack traces/stack references)
Check that a virtual thread is mounted is non-trivial, method from JvmtiEnvBase was used for this.

Testing: tier1..3, heapdump-related tests: open/test/hotspot/jtreg/serviceability,open/test/hotspot/jtreg/runtime/ErrorHandling,open/test/hotspot/jtreg/gc/epsilon,open/test/jdk/sun/tools/jhsdb

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Commit messages:
 - jcheck
 - JDK-8322237

Changes: https://git.openjdk.org/jdk/pull/17134/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=17134&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8322237
  Stats: 25 lines in 2 files changed: 23 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/17134.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/17134/head:pull/17134

PR: https://git.openjdk.org/jdk/pull/17134