RFR: 8338694: x86_64 intrinsic for tanh using libm [v2]

[Srinivas Vamsi Parasa]

> The goal of this PR is to implement an x86_64 intrinsic for java.lang.Math.tanh() using libm
> 
> Benchmark (ops/ms) | Stock JDK | Tanh intrinsic | Speedup
> -- | -- | -- | --
> MathBench.tanhDouble | 70900 | 95618 | 1.35x

Srinivas Vamsi Parasa has updated the pull request incrementally with one additional commit since the last revision:

  Add stub initialization and extra tanh tests

-------------

Changes:
  - all: https://git.openjdk.org/jdk/pull/20657/files
  - new: https://git.openjdk.org/jdk/pull/20657/files/79766f1b..4739ad45

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=20657&range=01
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=20657&range=00-01

  Stats: 65 lines in 2 files changed: 65 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/20657.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20657/head:pull/20657

PR: https://git.openjdk.org/jdk/pull/20657