RFR: 8263375: Support stack watermarks in Zero VM [v2]

[Aleksey Shipilev]

> Zero VM supports most of GCs. Since JDK 16, Shenandoah uses stack watermarks, so Zero has to support those if Shenandoah+Zero support is to remain. This PR adds the stack watermark support in Zero VM. This should also be useful as other projects, notably Loom, mature and depend on stack watermarks.
> 
> Zero already calls into Hotspot safepoint machinery to do things, and it seems only the hooks for `on_iteration` and `on_unwind` are missing. AFAICS, Zero only has on-return safepoints, renamed it to be more precise.
> 
> @fisk, do you see any obvious problems with this patch?
> 
> Additional testing:
>  - [x] Linux x86_64 Zero `hotspot_gc_shenandoah` now passes

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains four additional commits since the last revision:

 - Notify only when unwinding messages are received
 - Merge branch 'master' into JDK-8263375-zero-stack-watermarks
 - Revert debugging
 - 8263375: Support stack watermarks in Zero VM

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

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/4728/files
  - new: https://git.openjdk.java.net/jdk/pull/4728/files/d68b31a8..d809aad2

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=4728&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=4728&range=00-01

  Stats: 87518 lines in 1860 files changed: 67554 ins; 10358 del; 9606 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4728.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4728/head:pull/4728

PR: https://git.openjdk.java.net/jdk/pull/4728