cmd/go: quote command line arguments in debug output
cmd/go: quote command line arguments in debug output

Debug output from go test -x may contain empty arguments.
This CL quotes arguments if needed. E.g. the output of
go test -x is now
  .../6g -o ./_go_.6 -p testmain -complete -D "" -I . -I $WORK ./_testmain.go
which is easier to grasp.

R=golang-dev, bradfitz, minux.ma, r
CC=golang-dev
https://codereview.appspot.com/8633043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

gnanderson/go,dsriram/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,apparentlymart/golang,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,dsriram/golang,pombredanne/go-deleteme,apparentlymart/golang,gnanderson/go,apparentlymart/golang,gnanderson/go,tv42/old-go,apparentlymart/golang,gnanderson/go,pombredanne/go-deleteme,tv42/old-go,dsriram/golang,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,dsriram/golang,tv42/old-go,dsriram/golang,tv42/old-go,apparentlymart/golang