cmd/dist: make cc rule match what cmd/go uses
cmd/dist: make cc rule match what cmd/go uses

We added -I$GOROOT/pkg/$GOOS_$GOARCH in cmd/go
(I think for use by cgo and swig, primarily) but didn't
update cmd/dist. I was testing some other code and
found that my changes built with cmd/go but failed
during the initial bootstrap. Make them match again.

R=golang-dev, iant
CC=golang-dev
https://codereview.appspot.com/7707044

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