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

scirelli/scirelli-go,bryanxu/go-zh,webfd/go-zh,mhennings/marcohennings-go,mhennings/marcohennings-go,webfd/go-zh,mhennings/marcohennings-go,d0f/go-zh,glycerine/jeaten-go-arrayof-structof,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,sanjosh/sanjos100-tipc,bryanxu/go-zh,scirelli/scirelli-go,sanjosh/sanjos100-tipc,sanjosh/sanjos100-tipc,mhennings/marcohennings-go,webfd/go-zh,scirelli/scirelli-go,d0f/go-zh,mhennings/marcohennings-go,scirelli/scirelli-go,rdp/rogerpack2005-golang,webfd/go-zh,sanjosh/sanjos100-tipc,d0f/go-zh,webfd/go-zh,mhennings/marcohennings-go,bryanxu/go-zh,sanjosh/sanjos100-tipc,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,webfd/go-zh,rdp/rogerpack2005-golang,scirelli/scirelli-go,scirelli/scirelli-go,d0f/go-zh,rdp/rogerpack2005-golang,rdp/rogerpack2005-golang,mhennings/marcohennings-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,webfd/go-zh,glycerine/jeaten-go-arrayof-structof,glycerine/jeaten-go-arrayof-structof,d0f/go-zh,rdp/rogerpack2005-golang,d0f/go-zh,rdp/rogerpack2005-golang,webfd/go-zh,d0f/go-zh,sanjosh/sanjos100-tipc,bryanxu/go-zh,glycerine/jeaten-go-arrayof-structof,scirelli/scirelli-go,sanjosh/sanjos100-tipc,bryanxu/go-zh,bryanxu/go-zh,d0f/go-zh,mhennings/marcohennings-go,glycerine/jeaten-go-arrayof-structof