Also, tweak run.go to use no more than 2x the
number of CPUs, and only one on ARM.

53.85u 13.33s 53.69r 	 ./run
50.68u 12.13s 18.85r 	 go run run.go

Fixes #2833.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5754047