ARM asm for AV_RN*()

ARMv6 and later support unaligned loads and stores for single
word/halfword but not double/multiple.  GCC is ignorant of this and
will always use bytewise accesses for unaligned data.  Casting to an
int32_t pointer is dangerous since a load/store double or multiple
instruction might be used (this happens with some code in FFmpeg).
Implementing the AV_[RW]* macros with inline asm using only supported
instructions gives fast and safe unaligned accesses.  ARM RVCT does
the right thing with generic code.

This gives an overall speedup of up to 10%.

Originally committed as revision 18601 to svn://svn.ffmpeg.org/ffmpeg/trunk

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