go/build: temporarily disable cgo for freebsd/arm

Update #7331

cgo is currently broken on freebsd/arm.

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/63900043

src/pkg/go/build/build.go

@@ -264,7 +264,6 @@ var cgoEnabled = map[string]bool{
 	"dragonfly/amd64": true,
 	"freebsd/386":     true,
 	"freebsd/amd64":   true,
-	"freebsd/arm":     true,
 	"linux/386":       true,
 	"linux/amd64":     true,
 	"linux/arm":       true,