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

BenchmarkEqual32                       662          159  -75.98%
BenchmarkEqual4K                     76545        13719  -82.08%
BenchmarkEqual4M                  90136700     23588870  -73.83%
BenchmarkEqual64M               2147483647   1419616000  -42.63%

BenchmarkEqual32                     48.32       201.15    4.16x
BenchmarkEqual4K                     53.51       298.56    5.58x
BenchmarkEqual4M                     46.53       177.81    3.82x
BenchmarkEqual64M                    27.12        47.27    1.74x

R=golang-dev, qyzhai, minux.ma, rsc, iant, nigeltao
CC=golang-dev
http://codereview.appspot.com/6118049
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go