math: ARM assembly implementation for Abs
math: ARM assembly implementation for Abs

Obtained on 700MHz OMAP4460:
benchmark       old ns/op    new ns/op    delta
BenchmarkAbs           61           23  -61.63%

R=dave, remyoudompheng, mtj, rsc
CC=golang-dev
http://codereview.appspot.com/6094047

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