feat: add C implementation for math/base/special/floor2

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

• adds C implementation for math/base/special/floor2.

double stdlib_base_floor2( const double x );

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves a part of [RFC]: Add C implementations to base special math functions (tracking issue) #649.

Questions

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No.

Other

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No.

Checklist

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• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[Planeshifter]

LGTM!