fix: correctly handle signed zeroes in math/base/special/atanf

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

• modifies both C and JavaScript implementations to handle signed zeroes in math/base/special/atanf.
• adds corresponding test for checking results with positive and negative zeroes.

These changes are based on this.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves a part of [RFC]: add math/base/special/acotf #2113.

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.

• Read, understood, and followed the contributing guidelines.

---

@stdlib-js/reviewers

[kgryte]

LGTM. Thanks, @gunjjoshi!