RFR: 8329816: Add SLEEF version 3.6.1 [v3]

[Magnus Ihse Bursie]

On Thu, 27 Jun 2024 22:03:37 GMT, Mikael Vidstedt <mikael at openjdk.org> wrote:

>> [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

After thinking a bit more on this, I would recommend a somewhat different layout:


${JDK_ROOT}/src
  |-- jdk.incubator.vector/linux/native/libsleef
      |-- README.md
      |-- upstream/ 
           |-- ... # original sleef source (trimmed or not)
      |-- generated/
           |-- misc.h
           |-- sleefinline_advsimd.h
           |-- sleefinline_rvvm1.h
           |-- sleefinline_sve.h
  |-- jdk.incubator.vector/linux/legal/libsleef.md


`jdk.incubator.vector/linux/legal/libsleef.md` needs to contain the libsleef
license; see other legal/*.md files for reference.

The README.md file should briefly but clearly explain how we handle libsleef
code in the JDK, explain how to update the generated files, and describe
from where the source was obtained and at what git tag+hash.

In the build system, we will then add jdk.incubator.vector/linux/native/libsleef/generated as an additional source root when building libvectormath.

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PR Comment: https://git.openjdk.org/jdk/pull/19185#issuecomment-2317500698
PR Comment: https://git.openjdk.org/jdk/pull/19185#issuecomment-2317501858