cmd/dist: remove stray src/pkg/runtime/z* when cleaning
cmd/dist: remove stray src/pkg/runtime/z* when cleaning

R=golang-dev, bradfitz, dave, rsc, lstoakes, rsc
CC=golang-dev
http://codereview.appspot.com/6180044

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