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

[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)

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Commit messages:
 - RISC-V: Allow virtual thread preemption on some common class initialization paths

Changes: https://git.openjdk.org/loom/pull/222/files
  Webrev: https://webrevs.openjdk.org/?repo=loom&pr=222&range=00
  Stats: 186 lines in 11 files changed: 131 ins; 23 del; 32 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