This patch resulted from a bit of quick optimisation in response to a
golang-nuts post. It looks like one could save a couple other copies in
this function, but this addresses the inner loop and is fairly simple.

benchmark                    old ns/op    new ns/op    delta
BenchmarkGCD10x10                 1964         1711  -12.88%
BenchmarkGCD10x100                2019         1736  -14.02%
BenchmarkGCD10x1000               2471         2171  -12.14%
BenchmarkGCD10x10000              6040         5778   -4.34%
BenchmarkGCD10x100000            43204        43025   -0.41%
BenchmarkGCD100x100              11004         8520  -22.57%
BenchmarkGCD100x1000             11820         9446  -20.08%
BenchmarkGCD100x10000            23846        21382  -10.33%
BenchmarkGCD100x100000          133691       131505   -1.64%
BenchmarkGCD1000x1000           120041        95591  -20.37%
BenchmarkGCD1000x10000          136887       113600  -17.01%
BenchmarkGCD1000x100000         295370       273912   -7.26%
BenchmarkGCD10000x10000        2556126      2205198  -13.73%
BenchmarkGCD10000x100000       3159512      2808038  -11.12%
BenchmarkGCD100000x100000    150543094    139986045   -7.01%

R=gri, remyoudompheng
CC=bradfitz, gobot, golang-dev, gri
https://golang.org/cl/9424043