RFR: 8261473: Shenandoah: Add breakpoint suppoprt [v3]

[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

Zhengyu Gu has updated the pull request incrementally with one additional commit since the last revision:

  Shade's comments and fixing a merge error

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/2489/files
  - new: https://git.openjdk.java.net/jdk/pull/2489/files/2b88f7a6..4f19c4cd

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=2489&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=2489&range=01-02

  Stats: 18 lines in 3 files changed: 2 ins; 7 del; 9 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