RFR: 8261473: Shenandoah: Add breakpoint suppoprt

[Zhengyu Gu]

Please review this patch that adds breakpoint support for Shenandoah, that allows Shenandoah to access a few tests:

gc/TestConcurrentGCBreakpoints.java
gc/TestJNIWeak/TestJNIWeak.java
gc/TestReferenceClearDuringMarking.java
gc/TestReferenceClearDuringReferenceProcessing.java
gc/TestReferenceRefersTo.java

The drawback is that above tests can not run with passive mode, which can result tests to hang, as breakpoints only apply to concurrent GC.

Test:
- [x] hotspot_gc_shenandoah
- [x] tier1 with Shenandoah

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Commit messages:
 - update
 - init update

Changes: https://git.openjdk.java.net/jdk/pull/2489/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2489&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261473
  Stats: 170 lines in 8 files changed: 158 ins; 2 del; 10 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2489.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2489/head:pull/2489

PR: https://git.openjdk.java.net/jdk/pull/2489