8250246: Address reliance on default constructors in security libs

Reviewed-by: mullan

src/java.base/share/classes/java/security/SignatureSpi.java

@@ -52,6 +52,11 @@ import sun.security.jca.JCAUtil;
 
 public abstract class SignatureSpi {
 
+    /**
+     * Constructor for subclasses to call.
+     */
+    public SignatureSpi() {}
+
     /**
      * Application-specified source of randomness.
      */