RFR: 8339643: Port JEP 404 to RISC-V [v2]

[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 incrementally with one additional commit since the last revision:

  Review

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Changes:
  - all: https://git.openjdk.org/shenandoah/pull/493/files
  - new: https://git.openjdk.org/shenandoah/pull/493/files/ab573a27..9be141c0

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=493&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=493&range=00-01

  Stats: 1 line in 1 file changed: 0 ins; 0 del; 1 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