PPC is normally big endian but has special little endian load/store
instructions.  Using these avoids a separate byteswap.  This makes the
vorbis decoder about 5% faster.  Not much else uses little-endian
read/write extensively.

GCC generates horrible PPC code for the default AV_[RW]B64 (which uses
a packed struct), so we override it with a plain pointer cast.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@18602 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b