misc/dashboard/builder: handle Plan 9 in defaultSuffix()
misc/dashboard/builder: handle Plan 9 in defaultSuffix()

R=golang-dev, bradfitz
CC=golang-dev
https://codereview.appspot.com/9495045

apparentlymart/golang,pombredanne/go-deleteme,pombredanne/go-deleteme,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,apparentlymart/golang,gnanderson/go,tv42/old-go,tv42/old-go,apparentlymart/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,apparentlymart/golang,pombredanne/go-deleteme,gnanderson/go,tv42/old-go,gnanderson/go,gnanderson/go,pombredanne/go-deleteme,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang