RFR: 8315716: RISC-V: implement ChaCha20 intrinsic [v2]

[Hamlin Li]

> Only vector version is included in this patch.
> 
> ### Test
> The patch passed the jdk tests found via `find test/jdk/ -iname *ChaCha*`

Hamlin Li has updated the pull request incrementally with one additional commit since the last revision:

  revert adding t3-t6

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/15899/files
  - new: https://git.openjdk.org/jdk/pull/15899/files/100d7c61..fc19cb23

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=15899&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=15899&range=00-01

  Stats: 40 lines in 2 files changed: 9 ins; 7 del; 24 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