RFR: 8329816: Add SLEEF version 3.6.1

[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.

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Commit messages:
 - Expand tabs to 8 spaces to make jcheck happy
 -  Add generated sources for riscv64
 - Remove executable bit
 - Add generated sources for aarch64
 - Add build support for updating sleef sources
 - Add README
 - Add sleef license in legal
 - * Add source of libsleef 3.6.1

Changes: https://git.openjdk.org/jdk/pull/20781/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20781&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8329816
  Stats: 120301 lines in 175 files changed: 120301 ins; 0 del; 0 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