RFR: 8333300: [JVMCI] add support for generational ZGC [v3]

[Tom Rodriguez]

> This exposes the required values for JVMCI to support generational ZGC.  It includes a few things worth mentioning.  JVMCI still exports XBarrierSetRuntime as fields in CompilerToVM::Data under the original name of ZBarrierSetRuntime.  I have exported the XBarrierSetRuntime and ZBarrierSetRuntime functions as addresses under their actual name.  This permits backward compatibility until all the required parts are in place.  We can eventually delete the CompilerToVM::Data names.
> 
> I added ZBarrierSetRuntime::load_barrier_on_oop_array paralleling XBarrierSetRuntime::load_barrier_on_oop_array as we use that for a vector barrier.  I could create the function as part of JVMCIRuntime if there are any concerns about including that in the ZGC core.

Tom Rodriguez 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 ten additional commits since the last revision:

 - Merge branch 'tkr-genz' of github.com:tkrodriguez/jdk into tkr-genz
 - Fix riscv compilation
 - 8333300: [JVMCI] add support for generational ZGC
 - Merge branch 'master' into tkr-genz
 - Merge branch 'master' into tkr-genz
 - Use NativeAccess to read from handles
 - Enable support for UseEpsilonGC
 - Fix riscv compilation
 - 8333300: [JVMCI] add support for generational ZGC

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19490/files
  - new: https://git.openjdk.org/jdk/pull/19490/files/bb91b42c..8e0cf360

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19490&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19490&range=01-02

  Stats: 6305 lines in 231 files changed: 3674 ins; 1867 del; 764 mod
  Patch: https://git.openjdk.org/jdk/pull/19490.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19490/head:pull/19490

PR: https://git.openjdk.org/jdk/pull/19490