RFR: 8296602: RISC-V: improve performance of copy_memory stub

[Vladimir Kempik]

Please review this change to improve the performance of copy_memory stub on risc-v

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Commit messages:
 - 8296602: RISC-V: improve performance of copy_memory stub

Changes: https://git.openjdk.org/jdk/pull/11058/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=11058&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296602
  Stats: 63 lines in 1 file changed: 37 ins; 1 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/11058.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/11058/head:pull/11058

PR: https://git.openjdk.org/jdk/pull/11058