RFR: 8329816: Add SLEEF version 3.6.1 [v3]

[Mikael Vidstedt]

> [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 enhancement adds the relevant files, enabling the rest of [JDK-8312425](https://bugs.openjdk.org/browse/JDK-8312425) to move forward.

Mikael Vidstedt has updated the pull request incrementally with one additional commit since the last revision:

  Update README to include RISC-V

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Changes:
  - all: https://git.openjdk.org/jdk/pull/19185/files
  - new: https://git.openjdk.org/jdk/pull/19185/files/b29de559..8a08ffa1

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=19185&range=02
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=19185&range=01-02

  Stats: 10 lines in 1 file changed: 9 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jdk/pull/19185.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19185/head:pull/19185

PR: https://git.openjdk.org/jdk/pull/19185