Add optional NEON intrinsics to core/src/arithm.cpp

I wrote some vector-processing code using NEON compiler intrinsics, and in the future I plan to optimize it even more.

The performance comparisons on the same CPU - an Allwinner A10 (ARM Cortex A8 core) are here: http://pastebin.com/raw.php?i=VpJAHYgj