PPC64: Add libmvec SIMD single-precision cosine function [BZ #24205]

Implements single-precision cosine using VSX vector capability. The polynomial
cosine-approximating algorithm is adapted for PPC64 from x86_64 [commit #04f496d6].

The patch has been tested on PPC64/POWER8 Little Endian and Big Endian. It is
tested using the framework created for libmvec on x86_64 which runs tests on
issuing 'make check'. Tests of the new vector cosine function all pass.

Details on the ABI are found at this link:
<https://sourceware.org/glibc/wiki/


libmvec?action=AttachFile&do=view&target=VectorABI.txt>

But for adjusting the width of operands, details described for the
double-precision cosine implemented earlier apply here. See git
commit #7956c29f07 for that information.

Reviewed-by: Tulio Magno Quites Machado Filho <tuliom@linux.ibm.com>