RFR: 8329816: Add SLEEF version 3.6.1 [v6]

[Magnus Ihse Bursie]

> [JDK-8312425](https://bugs.openjdk.org/browse/JDK-8312425) is looking to optimize vector math operations by leveraging the SLEEF library. For legal reasons the actual contribution of the SLEEF files needs to be handled separately.
> 
> This is a new attempt at solving [JDK-8329816](https://bugs.openjdk.org/browse/JDK-8329816); the original attempt is here: https://github.com/openjdk/jdk/pull/19185. This PR is based on the discussions on how to move forward that was held in that original PR.

Magnus Ihse Bursie has updated the pull request incrementally with one additional commit since the last revision:

  Update legal/sleef.md

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Changes:
  - all: https://git.openjdk.org/jdk/pull/20781/files
  - new: https://git.openjdk.org/jdk/pull/20781/files/e5fe681e..83dddf1f

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20781&range=05
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20781&range=04-05

  Stats: 407 lines in 1 file changed: 404 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/20781.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20781/head:pull/20781

PR: https://git.openjdk.org/jdk/pull/20781