PPC64: Add libmvec SIMD double-precision sincos function [BZ #24207]

Implements double-precision vector sincos function. The polynomial approxima-
ting algorithm is adapted for PPC64 from x86_64 [commit #c9a8c526].

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

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