[riscv-port-jdk17u:riscv-port] RFR: 8301818: RISC-V: Factor out function mvw from MacroAssembler

[Gui Cao]

Hi,
Please help review this backport to riscv-port-jdk17u.
Backport of [JDK-8301818](https://bugs.openjdk.org/browse/JDK-8301818), and [JDK-8302114](https://bugs.openjdk.org/browse/JDK-8302114).
Because there is no [JDK-8286301](https://bugs.openjdk.org/browse/JDK-8286301), [JDK-8290154](https://bugs.openjdk.org/browse/JDK-8290154) here in riscv-port-jdk17u, it will show not clean.

Testing:

Tier1-3 passed without new failure on unmacthed (release).

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Commit messages:
 - backport 8301818: RISC-V: Factor out function mvw from MacroAssembler

Changes: https://git.openjdk.org/riscv-port-jdk17u/pull/62/files
 Webrev: https://webrevs.openjdk.org/?repo=riscv-port-jdk17u&pr=62&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8301818
  Stats: 30 lines in 9 files changed: 0 ins; 2 del; 28 mod
  Patch: https://git.openjdk.org/riscv-port-jdk17u/pull/62.diff
  Fetch: git fetch https://git.openjdk.org/riscv-port-jdk17u.git pull/62/head:pull/62

PR: https://git.openjdk.org/riscv-port-jdk17u/pull/62