Fix cross compile for make cross

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@crosbymichael.com> (github: crosbymichael)

system/unsupported.go

@@ -0,0 +1,15 @@
+// +build !linux
+
+package system
+
+import (
+	"os/exec"
+)
+
+func SetCloneFlags(cmd *exec.Cmd, flag uintptr) {
+
+}
+
+func UsetCloseOnExec(fd uintptr) error {
+	return ErrNotSupportedPlatform
+}