gnu: libvpx: Apply bug fixes.

Fixes build on armhf.

* gnu/packages/patches/libvpx-fix-armhf-link.patch,
  gnu/packages/patches/libvpx-fix-ssse3-quantize.patch,
  gnu/packages/patches/libvpx-vp9-out-of-bounds-access.patch: New files.
* gnu-system.am (dist_patch_DATA): Add them.
* gnu/packages/video.scm (libvpx): Add patches.  On armhf, add additional
  configure flags to avoid requiring support for NEON.