PPC64: Add libmvec SIMD double-precision logarithm function [BZ #24208]

Implements double-precision vector logarithm function.  The algorithm is
an adaptation of the one in sysdeps/ieee754/dbl-64, modified to exploit
PPC64 VSX hardware.  The version of ieee754/dbl-64 is commit #f41b0a43.

The patch has been tested on both Little-Endian and Big-Endian.  It
passes all the tests for double-precision logarithm run by make check.
Integration into the make check infrastructure closely follows corres-
ponding changes done for x86_64 in commit #6af25acc.

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