cmd/go: fix invalid go pack command line in the output of go build -x
cmd/go: fix invalid go pack command line in the output of go build -x

Fixes issue 7262

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://codereview.appspot.com/76110043

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