Implement loadavg on FreeBSD without cgo

The code may also work for other BSDs, but I don't have access to those
for testing.

collector/loadavg_freebsd.go

@@ -0,0 +1,27 @@
+// +build !noloadavg
+
+package collector
+
+import (
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+func getLoad() ([]float64, error) {
+	type loadavg struct {
+		load  [3]uint32
+		scale int
+	}
+	b, err := unix.SysctlRaw("vm.loadavg")
+	if err != nil {
+		return nil, err
+	}
+	load := *(*loadavg)(unsafe.Pointer((&b[0])))
+	scale := float64(load.scale)
+	return []float64{
+		float64(load.load[0]) / scale,
+		float64(load.load[1]) / scale,
+		float64(load.load[2]) / scale,
+	}, nil
+}

collector/loadavg_unix.go

@@ -11,7 +11,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build darwin dragonfly freebsd netbsd openbsd solaris
+// +build darwin dragonfly netbsd openbsd solaris
 // +build !noloadavg
 
 package collector