runtime: in newstack, double-check calling goroutine

Checking this condition helped me find the arm problem last night.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12267043
testing: make parsing of -cpu more lenient

Also add \n to error message.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/12261044