PPC64 libmvec sin, cos, sincos

[GT <tnggil at protonmail dot com>]

Here is a suggestion to organize the implementation of the three functions
a little differently than was done for x86_64.

Because of the trigonometric identity cos (x) = sin( x + PI/2), selecting
either the sine or cosine polynomial series approximation should suffice to
compute all three functions.

Defining the series in a static inline function will let us call it with
input argument x or x +/- PI/2 for sine and cosine (depending on which of the
two series was chosen). For sincos, the function will be called twice: once
to obtain the cosines of the inputs and a second time to get the sines.

Doing that will allow us to maintain a single file which performs evaluations
for the three functions.

Any reason not to implement in this style?

Thanks.
Bert.