[jdk21u] RFR: 8314618: RISC-V: -XX:MaxVectorSize does not work as expected

[Gui Cao]

Hi, I would like to backport this to jdk21u Make -XX:MaxVectorSize work correctly on Linux RISC-V platform.
`test/hotspot/jtreg/compiler/loopopts/superword/TestUnorderedReduction.java` passed with fastdebug and use build using qemu.

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Commit messages:
 - Backport a66b5df14a163d2990e6dd746906942367e4fdb2

Changes: https://git.openjdk.org/jdk21u/pull/80/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u&pr=80&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8314618
  Stats: 4 lines in 2 files changed: 1 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u/pull/80.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u.git pull/80/head:pull/80

PR: https://git.openjdk.org/jdk21u/pull/80