RFR: 8309502: RISC-V: String.indexOf intrinsic may produce misaligned memory loads

[Vladimir Kempik]

Clean backport to jdk21,  fixes misaligned access in string.IndexOf intrinsic, significantly improves performance of intrinsic on hardware without misaligned access support

-------------

Commit messages:
 - Backport 6b942893868fa1a64977288bdbdb1bbff8bd9d9c

Changes: https://git.openjdk.org/jdk21/pull/20/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21&pr=20&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8309502
  Stats: 84 lines in 3 files changed: 67 ins; 5 del; 12 mod
  Patch: https://git.openjdk.org/jdk21/pull/20.diff
  Fetch: git fetch https://git.openjdk.org/jdk21.git pull/20/head:pull/20

PR: https://git.openjdk.org/jdk21/pull/20