ppc: disable vsx optimizations with hbd
ppc: disable vsx optimizations with hbd

vsx optimizations do not support 32 bit tran_low_t values.

BUG=webm:1563

Change-Id: I9e6348078f6e4855acfd381133eb840a435b7f81

ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx,webmproject/libvpx,ShiftMediaProject/libvpx