RFR: 8307378: Allow collectors to provide specific values for GC notifications' actions [v3]

[William Kemper]

> At the end of a GC pause, a `GarbageCollectionNotificationInfo` may be emitted. The notification has a `gcAction` field which presently originates from the field `_gc_end_message` in `GCMemoryManager`. Concurrent collectors such as Shenandoah, ZGC and G1 may have more (brief) pauses in their cycle than they have memory managers. This makes it difficult for gc notification listeners to determine the phase of the cycle that emitted the notification. We are proposing a change to allow collectors to define specific values for the `gcAction` to make it easier for notification listeners to classify the gc phase responsible for the notification.

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Remove trailing whitespace

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

Changes:
  - all: https://git.openjdk.org/jdk/pull/13785/files
  - new: https://git.openjdk.org/jdk/pull/13785/files/a85685b8..918c20cf

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=13785&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=13785&range=01-02

  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/13785.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/13785/head:pull/13785

PR: https://git.openjdk.org/jdk/pull/13785