RFR: 8269404: Base64 Encoding optimization enhancements for x86 using AVX-512

[Scott Gibbons]

Enhance the Base64 Encode intrinsic for x86 using AVX-512 to get better performance. Also allow for performance improvement on non-AVX-512 enabled platforms.

Added AVX-512 code for encoding Base64 blocks, including slight improvements for non-AVX-512 x86 platforms.

Running the Base64Decode benchmark, this change increases decode performance by an average of 2.6x with a maximum 9.7x for buffers > ~20k.  The numbers are given in the table below.

**Base Score** is without intrinsic support, **Optimized Score** is using this intrinsic, and **Gain** is **Base** / **Optimized**.

Benchmark Name | Base Score | Optimized Score | Gain
-- | -- | -- | --
testBase64Encode size 1 | 8.10 | 8.04 | 1.01
testBase64Encode size 2 | 8.51 | 8.43 | 1.01
testBase64Encode size 3 | 11.08 | 10.72 | 1.03
testBase64Encode size 6 | 13.98 | 13.12 | 1.07
testBase64Encode size 7 | 14.44 | 13.38 | 1.08
testBase64Encode size 9 | 15.44 | 14.37 | 1.07
testBase64Encode size 10 | 16.13 | 14.97 | 1.08
testBase64Encode size 48 | 27.14 | 23.23 | 1.17
testBase64Encode size 512 | 123.86 | 30.75 | 4.03
testBase64Encode size 1000 | 224.42 | 37.71 | 5.95
testBase64Encode size 20000 | 4202.11 | 430.16 | 9.77

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Commit messages:
 - Whitespace cleanup.
 - Windows build warnings removal.
 - Revert name change for decode.
 - Formatting change; forgotten removal of add to dp.
 - Adding forgotten assembly instructions.
 - Adding forgotten assembly instructions.
 - Adding forgotten assembly instructions.
 - First cut at Base64 Encoder.

Changes: https://git.openjdk.java.net/jdk/pull/4601/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=4601&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8269404
  Stats: 707 lines in 5 files changed: 348 ins; 233 del; 126 mod
  Patch: https://git.openjdk.java.net/jdk/pull/4601.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/4601/head:pull/4601

PR: https://git.openjdk.java.net/jdk/pull/4601