16 seems pretty standard on x86 for function entry.
I don't know if ARM would benefit, so I used just 4
(single instruction alignment).

This has a minor absolute effect on the current timings.
The main hope is that it will make them more consistent from
run to run.

benchmark                 old ns/op    new ns/op    delta
BenchmarkBinaryTree17    4222117400   4140739800   -1.93%
BenchmarkFannkuch11      3462631800   3259914400   -5.85%
BenchmarkGobDecode         20887622     20620222   -1.28%
BenchmarkGobEncode          9548772      9384886   -1.72%
BenchmarkGzip                151687       150333   -0.89%
BenchmarkGunzip                8742         8741   -0.01%
BenchmarkJSONEncode        62730560     65210990   +3.95%
BenchmarkJSONDecode       252569180    249394860   -1.26%
BenchmarkMandelbrot200      5267599      5273394   +0.11%
BenchmarkRevcomp25M       980813500    996013800   +1.55%
BenchmarkTemplate         361259100    360620840   -0.18%

R=ken2
CC=golang-dev
https://golang.org/cl/6244066