update ffmpeg/config.h
include MIPS headers only when ARCH_MIPS is defined
remove the unneeded MIPS files
update ffmpeg (n0.8-19337-g5f8f9dc)

Notes: broken atm because our custom files need to be updated too

* libavcodec/h264_dxva.c
* libavcodec/h264_recov.c
* libavcodec/vc1_dxva.c