go/build: disable cgo on freebsd/arm

Under FreeBSD-CURRENT on arm, cgo enabled binaries segfault. Disable cgo support for the moment so we can have a freebsd/arm builder on the dashboard.

R=minux.ma, rsc, iant
CC=golang-dev
https://golang.org/cl/7031044

src/pkg/go/build/build.go

@@ -216,7 +216,6 @@ var cgoEnabled = map[string]bool{
 	"darwin/amd64":  true,
 	"freebsd/386":   true,
 	"freebsd/amd64": true,
-	"freebsd/arm":   true,
 	"linux/386":     true,
 	"linux/amd64":   true,
 	"linux/arm":     true,