add freebsd to the list of os'es. make the GOOS/GOARCH pair a table to avoid the confusion caused when it was of the form linux/386.
add freebsd to the list of os'es.
make the GOOS/GOARCH pair a table to avoid the confusion caused when it was of the form linux/386.

R=rsc
CC=golang-dev
http://codereview.appspot.com/160061

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