RFR: 8377223: Port fdlibm atanh to Java

[Anton Artemov]

Hi, please consider the following changes:

This is a port of FDLIBM atanh method.

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Commit messages:
 - 8377223: Fixed typos.
 - 8377223: Fixed whitespaces.
 - 8377223: Fixed whitespaces.
 - 8377223: Removed commented out line.
 - 8377223: Port fdlibm atanh to java.

Changes: https://git.openjdk.org/jdk/pull/29782/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29782&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8377223
  Stats: 652 lines in 7 files changed: 646 ins; 0 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/29782.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29782/head:pull/29782

PR: https://git.openjdk.org/jdk/pull/29782