misc/cgo/test: do not run wasteCPU during TestParallelSleep unless on arm
misc/cgo/test: do not run wasteCPU during TestParallelSleep unless on arm

R=golang-dev, r, minux.ma, dave, bradfitz
CC=golang-dev
http://codereview.appspot.com/6532052

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang