RFR: 8355239: RISC-V: Do not support subword scatter store

[Fei Yang]

Hi, please consider this small enhancement change.

Currently, only word and double-word gather load and scatter store are supported on riscv.
Both subword gather load and scatter store are not supported due to constraint of riscv vector extension.
[JDK-8331150](https://bugs.openjdk.org/browse/JDK-8331150) makes this constraint explicit for subword gather load.
For parity and consistency, this also makes it explicit for subword scatter store as well.

Testing: `jdk_vector` tested with QEMU vector extension.

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Commit messages:
 - 8355239: RISC-V: Do not support subword scatter store

Changes: https://git.openjdk.org/jdk/pull/24787/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24787&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8355239
  Stats: 2 lines in 1 file changed: 2 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/24787.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24787/head:pull/24787

PR: https://git.openjdk.org/jdk/pull/24787