RFR: 8261759: ZGC: ZWorker Threads Continue Marking After System.exit() called [v3]

[Per Liden]

> During JVM termination, ZCollectedHeap::stop() currently only returns once any ongoing GC cycle has completed. However, waiting for a GC cycle to complete can take awhile, so the JVM might not terminate as promptly as one might expect.
> 
> This patch adds abortion checks to workers doing marking and relocation to allow ZWorker threads to terminate promptly. In addition, before and after concurrent phases, the ZDriver thread checks if `should_terminate()` is true and prematurely terminates the GC cycle if that's the case.
> 
> Testing:
>  * Tier1-7 on Linux.
>  * SPECjbb2015, comparing score and marking times against baseline (5 iterations each), no difference observed.
>  * Ad-hoc programs calling System.exit() during marking/relocation.
>  * Manual Ctrl-C testing.

Per Liden has updated the pull request incrementally with one additional commit since the last revision:

  Forward in-place if GC aborted

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/3453/files
  - new: https://git.openjdk.java.net/jdk/pull/3453/files/73603d6d..d0e6a56a

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=3453&range=02
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=3453&range=01-02

  Stats: 51 lines in 5 files changed: 36 ins; 9 del; 6 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3453.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3453/head:pull/3453

PR: https://git.openjdk.java.net/jdk/pull/3453