[ffmpeg] Backport of armv6 and vfp optmisations for DTS

There have been a sequence of optimisations accepted to upstream libavcodec/ffmpeg designed to make software decode of DTS run better on the Raspberry Pi.

Since DTS is a floating point codec, the optimisations mostly rely upon hand-scheduled VFP code and the use
of short vectors.

Overall this reduces the time spent in DTS by about 36%. This makes software DTS decode on Pi much more feasible without overclocking.

These patches can be dropped when ffmpeg is updated to a sufficiently new version. These patches aren't present in v.2.0, which may be the next logical upgrade step for ffmpeg, so I'd like to get these cherry-picked, rather than waiting for an ffmpeg bump.

The patches have been present in rbej builds for a few weeks and no regressions have been reported.