RFR: 8339643: Port JEP 404 to RISC-V

[Fei Yang]

The riscv64 implementation of [JDK-8337511](https://bugs.openjdk.org/browse/JDK-8337511) Implement JEP-404: Generational Shenandoah (Experimental)

Testing
- [x] Gtest (release & fastdebug)
- [x] hotspot_gc_shenandoah (release & fastdebug)

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Commit messages:
 - JDK-8339643: Port JEP 404 to RISC-V

Changes: https://git.openjdk.org/shenandoah/pull/493/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=493&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339643
  Stats: 90 lines in 4 files changed: 79 ins; 4 del; 7 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