A64: Add Step

Allow for stepping instruction-by-instruction

include/dynarmic/A64/a64.h

@@ -30,6 +30,12 @@ public:
      */
     void Run();
 
+    /**
+     * Step the emulated CPU for one instruction.
+     * Cannot be recursively called.
+     */
+    void Step();
+
     /**
      * Clears the code cache of all compiled code.
      * Can be called at any time. Halts execution if called within a callback.