RFR: 8286367: riscv: riscv port is broken after JDK-8284161

[Yadong Wang]

[JDK-8284161](https://bugs.openjdk.java.net/browse/JDK-8284161) implements a preview version of virtual threads on x86_64 and aarch64, and riscv port is broken everywhere. The initial patch is required to make everything on riscv work without "--enable-preview".

Hotspot/jdk tier1 and all jtreg cases have been tested on Unmatched and Qemu respectively without new failures, except for the "-enable-preview" ones.

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Commit messages:
 - 8286367: riscv: riscv port is broken after JDK-8284161

Changes: https://git.openjdk.java.net/jdk/pull/8595/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=8595&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8286367
  Stats: 183 lines in 7 files changed: 150 ins; 18 del; 15 mod
  Patch: https://git.openjdk.java.net/jdk/pull/8595.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/8595/head:pull/8595

PR: https://git.openjdk.java.net/jdk/pull/8595