bytes: add assembly version of IndexByte for ARM
bytes: add assembly version of IndexByte for ARM

benchmark                        old ns/op    new ns/op    delta
BenchmarkIndexByte32                   459          126  -72.55%
BenchmarkIndexByte4K                 52404        10939  -79.13%
BenchmarkIndexByte4M              54470800     11177370  -79.48%
BenchmarkIndexByte64M           1010803000    178860500  -82.31%

benchmark                         old MB/s     new MB/s  speedup
BenchmarkIndexByte32                 69.58       252.63    3.63x
BenchmarkIndexByte4K                 78.16       374.42    4.79x
BenchmarkIndexByte4M                 77.00       375.25    4.87x
BenchmarkIndexByte64M                66.39       375.20    5.65x

R=rsc, minux.ma
CC=golang-dev
http://codereview.appspot.com/6106044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go