RFR: 8351140: RISC-V: Intrinsify Unsafe::setMemory

[Anjian Wen]

>From [JDK-8329331](https://bugs.openjdk.org/browse/JDK-8329331), add riscv unsafe::setMemory intrinsic’s generator generate_unsafe_setmemory. This intrinsic optimizes about 15%-20% unsafe setmemory time

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Commit messages:
 - RISC-V: Intrinsify Unsafe::setMemory
 - RISC-V: Intrinsify Unsafe::setMemory

Changes: https://git.openjdk.org/jdk/pull/23890/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23890&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351140
  Stats: 49 lines in 1 file changed: 49 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23890.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23890/head:pull/23890

PR: https://git.openjdk.org/jdk/pull/23890