runtime: faster segmented stacks
benchmark                      old ns/op    new ns/op    delta
BenchmarkStackGrowth                 665          548  -17.59%
BenchmarkStackGrowth-2               333          274  -17.72%
BenchmarkStackGrowth-4               224          168  -25.00%
BenchmarkStackGrowth-8               124           91  -26.21%
BenchmarkStackGrowth-16               82           70  -14.55%
BenchmarkStackGrowth-32               73           59  -19.49%

R=nigeltao, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7026044