cmd/cgo: omit //line in -godefs, -cdefs output
cmd/cgo: omit //line in -godefs, -cdefs output

Makes files like src/pkg/syscall/ztypes_linux_amd64.go easier to read.
(The copy that is checked in predates the //line output mode,
so this also preserves the status quo.)

R=golang-dev, iant, gri
CC=golang-dev
http://codereview.appspot.com/5655068

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