refactor: update `math/base/special/atanh` according to the FreeBSD implementation

[Neerajpathak07]

Resolves #2089.

Description

What is the purpose of this pull request?

This pull request:

• Refactors the C implementation for math/base/special/atanh according to the FreeBSD implementation.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves #2089

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

No.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• [x] Read, understood, and followed the contributing guidelines

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@stdlib-js/reviewers

[Neerajpathak07]

Had to rebase due to addition of an unwanted change.

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
math/base/special/atanh	$\color{green}248/248$ $\color{green}+100.00\%$	$\color{green}24/24$ $\color{green}+100.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{green}248/248$ $\color{green}+100.00\%$

The above coverage report was generated for the changes in this PR.

[Neerajpathak07]

@kgryte Have made the changes in the C implementation according to the issue description. Will be working on the javascript implementation once you will green-light this!!.

[Neerajpathak07]

cc:- @gunjjoshi

[anandkaranubc]

/stdlib merge