RFR: 8296875: Generational ZGC: Refactor loom code [v6]

[Erik Österlund]

> The current loom code makes some assumptions about GC that will not work with generational ZGC. We should make this code more GC agnostic, and provide a better interface for talking to the GC.
> 
> In particular,
> 1) All GCs have a way of encoding oops inside of the heap differently to oops outside of the heap. For non-ZGC collectors, that is compressed oops. For ZGC, that is colored pointers. With generational ZGC, pointers on-heap will be colored and pointers off-heap will be "colorless". So we need to generalize encoding and decoding of oops in the heap, for loom.
> 
> 2) The cont_oop is located on a stack. In order to access it we need to start_processing on that thread, if it isn't the current thread. This happened to work so far for ZGC, because the stale pointers had enough colors. But with generational ZGC, these on-stack oops will be colorless, so we have to be more accurate here and ensure processing really has started on any thread that cont_oop is used on. To make life a bit easier, I'm moving the oop processing responsibility for these oops to the thread instead. Currently there is no more than one of these, so doing it lazily per frame seems a bit overkill.
> 
> 3) Refactoring the stack chunk allocation code
> 
> Tested with tier1-5 and manually running Skynet. No regressions detected. We have also been running with this (yet a slightly different backend) in the generational ZGC repo for a while now.

Erik Österlund has updated the pull request with a new target base due to a merge or a rebase. The pull request now contains seven commits:

 - PPC support
 - Merge branch 'master' into 8296875_refactor_loom_code
 - Patricio concerns
 - Fix Richard comments
 - Indentation fix
 - Fix verification and RISC-V support
 - Generational ZGC: Loom support

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

Changes: https://git.openjdk.org/jdk/pull/11111/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11111&range=05
  Stats: 978 lines in 42 files changed: 641 ins; 228 del; 109 mod
  Patch: https://git.openjdk.org/jdk/pull/11111.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11111/head:pull/11111

PR: https://git.openjdk.org/jdk/pull/11111