RFR: 8345298: RISC-V: Add riscv backend for Float16 operations - scalar

[Hamlin Li]

Hi,
Can you help to review this patch?
It's an implementation of https://github.com/openjdk/jdk/pull/22754 on riscv.

## Performance
still in progress ...

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Commit messages:
 - merge master
 - clean 2
 - clean
 - initial commit

Changes: https://git.openjdk.org/jdk/pull/23844/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23844&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8345298
  Stats: 447 lines in 11 files changed: 393 ins; 0 del; 54 mod
  Patch: https://git.openjdk.org/jdk/pull/23844.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23844/head:pull/23844

PR: https://git.openjdk.org/jdk/pull/23844