RFR: 8247645: ChaCha20 intrinsics [v6]

[Jamil Nimeh]

> This PR delivers ChaCha20 intrinsics that accelerate the core block function that generates key stream from the key, counter and nonce.  Intrinsics have been written for the following platforms and instruction sets:
> 
> - x86_64: AVX, AVX2 and AVX512
> - aarch64: platforms that support the advanced SIMD instructions
> 
> Note: Microbenchmark results moved to a comment in the PR so we don't have to see it in every email.
> 
> Special thanks to the folks who have made many helpful comments while this PR was in draft form.

Jamil Nimeh has updated the pull request incrementally with one additional commit since the last revision:

  Split comment paragraph up for readability/clarity

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Changes:
  - all: https://git.openjdk.org/jdk/pull/7702/files
  - new: https://git.openjdk.org/jdk/pull/7702/files/bb3f4264..b818411b

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=7702&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=7702&range=04-05

  Stats: 10 lines in 1 file changed: 4 ins; 3 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/7702.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/7702/head:pull/7702

PR: https://git.openjdk.org/jdk/pull/7702