cmd/dist: add BSD gohostos cases
cmd/dist: add BSD gohostos cases

Should fix FreeBSD build.

TBR=golang-dev
CC=golang-dev
http://codereview.appspot.com/5629050

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