RFR: 8339643: Port JEP 404 to RISC-V [v3]

[Fei Yang]

> The riscv64 implementation of [JDK-8337511](https://bugs.openjdk.org/browse/JDK-8337511) Implement JEP-404: Generational Shenandoah (Experimental).
> This is mainly adapted from the changes in aarch64.
> 
> Testing
> - [x] Gtest (release & fastdebug)
> - [x] hotspot_gc_shenandoah (release & fastdebug)

Fei Yang 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 three additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/master' into JDK-8339643
 - Review
 - JDK-8339643: Port JEP 404 to RISC-V

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/493/files
  - new: https://git.openjdk.org/shenandoah/pull/493/files/9be141c0..54c5f73b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=493&range=02
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=493&range=01-02

  Stats: 45277 lines in 1364 files changed: 27475 ins; 10650 del; 7152 mod
  Patch: https://git.openjdk.org/shenandoah/pull/493.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/493/head:pull/493

PR: https://git.openjdk.org/shenandoah/pull/493