RFR: 8350960: RISC-V: Add riscv backend for Float16 operations - vectorization

[Hamlin Li]

Hi,
Can you help to review this patch?
It's a follow-up of https://github.com/openjdk/jdk/commit/9a3f9997b68a1f64e53b9711b878fb073c3c9b90.
Thanks!

## Test

performance test running in progress ...

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Commit messages:
 - assert
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/25181/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=25181&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8350960
  Stats: 154 lines in 3 files changed: 142 ins; 1 del; 11 mod
  Patch: https://git.openjdk.org/jdk/pull/25181.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/25181/head:pull/25181

PR: https://git.openjdk.org/jdk/pull/25181