RFR: RISC-V: Allow virtual thread preemption on some common class initialization paths [v2]

[Fei Yang]

> This adds RISC-V support for : https://bugs.openjdk.org/browse/JDK-8369238
> 
> Testing performed on linux-riscv64 platform:
> - [x] hs:tier1 - hs:tier3 (release)
> - [x] make test TEST="hotspot_loom jdk_loom" (release)
> - [x] make test TEST="hotspot_loom jdk_loom" TEST_VM_OPTS="-XX:+VerifyStack -XX:+VerifyContinuations" (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 two additional commits since the last revision:

 - Merge remote-tracking branch 'upstream/fibers' into 8369238-riscv64
 - RISC-V: Allow virtual thread preemption on some common class initialization paths

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

Changes:
  - all: https://git.openjdk.org/loom/pull/222/files
  - new: https://git.openjdk.org/loom/pull/222/files/6f1bb94a..0e1b94f9

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=loom&pr=222&range=01
 - incr: https://webrevs.openjdk.org/?repo=loom&pr=222&range=00-01

  Stats: 3512 lines in 40 files changed: 3199 ins; 156 del; 157 mod
  Patch: https://git.openjdk.org/loom/pull/222.diff
  Fetch: git fetch https://git.openjdk.org/loom.git pull/222/head:pull/222

PR: https://git.openjdk.org/loom/pull/222