[libc] Fix couple of corner cases in remquo.

These two cases are fixed:
1. If numerator is not zero and denominator is infinity, then the
numerator is returned as the remainder.
2. If numerator and denominator are equal in magnitude, then quotient
with the right sign is returned.

The differet tests of remquo, remquof and remquol have been unified
into a single file to avoid duplication.

Reviewed By: lntue

Differential Revision: https://reviews.llvm.org/D92353