cmd/go: implement -x correctly for 'go vet', 'go fmt', and so on
cmd/go: implement -x correctly for 'go vet', 'go fmt', and so on

Fixes issue 5676.

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/13250047

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