cmd/cgo: fix build (missing file from earlier CL)

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/13700045
runtime: fix freebsd build

TBR=golang-dev
CC=golang-dev
https://codereview.appspot.com/13720044
cmd/gc: fix divide by zero error in compiler

Fixes issue 6399.

R=ken2
CC=golang-dev
https://codereview.appspot.com/13253055