go/build: disable cgo by default for freebsd/arm

Updates #10119

Temporarily disable cgo by default to get the freebsd/arm
builder running again.

Change-Id: I4de1f896fcac650184df77c692b102ea6fb73bba
Reviewed-on: https://go-review.googlesource.com/7125Reviewed-by: Andrew Gerrand <adg@golang.org>

src/go/build/build.go

@@ -263,7 +263,6 @@ 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,