[libc][math] Fix exact cases for powf. (#91488)

It was reported from the CORE-MATH project that the `powf`
implementation did not round correctly when `x^y` is either exact for
exactly half-way.

This PR deals with the potential exact cases when `y` is an integer `2 <
y < 25`. In such cases, the results of `x^y` is exactly representable in
double precision.