RFR: 8315716: RISC-V: implement ChaCha20 intrinsic

[Hamlin Li]

Only vector version is included in this patch.

### Test
The patch passed the jdk tests found via `find test/jdk/ -iname *ChaCha*`

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Commit messages:
 - change from li to mv to follow code convention
 - fix according round 2 reviewing
 - use mu/tu rather than ma/ta when calling vsetvli
 - fix according round 1 reviewing
 - Initial commit: chacha20 intrinsic implementation in vector instructions on riscv

Changes: https://git.openjdk.org/jdk/pull/15899/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=15899&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8315716
  Stats: 142 lines in 4 files changed: 142 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/15899.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/15899/head:pull/15899

PR: https://git.openjdk.org/jdk/pull/15899