go.tools/dashboard: Turn off multilib compilation in gccgo.
go.tools/dashboard: Turn off multilib compilation in gccgo.

R=adg
CC=golang-dev
https://codereview.appspot.com/43020043

lattera/go.tools,lattera/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,karmakaze/go.tools,karmakaze/go.tools,chenjianlong/go.tools,lattera/go.tools