[jdk19u] RFR: 8296602: RISC-V: improve performance of copy_memory stub

[Vladimir Kempik]

Clean backport to jdk19u
test results to follow

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Commit messages:
 - Backport bd57e2138fc980822a149af905e572ab71ccbf11

Changes: https://git.openjdk.org/jdk19u/pull/77/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk19u&pr=77&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8296602
  Stats: 70 lines in 1 file changed: 41 ins; 1 del; 28 mod
  Patch: https://git.openjdk.org/jdk19u/pull/77.diff
  Fetch: git fetch https://git.openjdk.org/jdk19u pull/77/head:pull/77

PR: https://git.openjdk.org/jdk19u/pull/77