swscale/ppc: VSX-optimize hScale16To*

swscale/ppc: VSX-optimize hScale16To*

./ffmpeg -loop 1 -s 1200x1440 -i tux16.png \
    -s 2400x720 -f rawvideo -y -vframes 5 -pix_fmt yuv420p16le -nostats test.raw

./ffmpeg -loop 1 -s 1200x1440 -i tux16.png \
    -s 2400x720 -f rawvideo -y -vframes 5 -pix_fmt yuv420p -nostats test.raw

32-bit mul, power8 only

2x speedup for hScale8To19_vsx (x86 SSE2 is 2.37):
  30896 UNITS in hscale,    8192 runs,      0 skips
  63956 UNITS in hscale,    8192 runs,      0 skips

2.06 for hScale16To15_vsx:
  30531 UNITS in hscale,    8192 runs,      0 skips
  63161 UNITS in hscale,    8192 runs,      0 skips