RFR: 8266744: Make AbstractGangTask stack-allocatable only

[Albert Mingkun Yang]

Remove the inheritance relation in `class AbstractGangTask : public CHeapObj<mtInternal>`, and export necessary classes.

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Commit messages:
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Changes: https://git.openjdk.java.net/jdk/pull/3918/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3918&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8266744
  Stats: 77 lines in 5 files changed: 23 ins; 24 del; 30 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3918.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3918/head:pull/3918

PR: https://git.openjdk.java.net/jdk/pull/3918