[libc] Include nextafter's functions to Windows's entrypoints

Incorporated the varied functions for nextafter and refactored
NextAfterTest.h to correctly define bitWidthOfType for both
Linux and Windows; by letting FloatProperties take care
of the directives' logic based on the platform being used.
This allows to successfully run nextafter's tests.

Reviewed By: sivachandra

Differential Revision: https://reviews.llvm.org/D106395