dashboard/builder: pass GO386 to child builder, like GOARM
dashboard/builder: pass GO386 to child builder, like GOARM

Update Issue 7720

LGTM=dave
R=adg, dave
CC=golang-codereviews, rsc
https://codereview.appspot.com/84410046

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