vm_call_method: use struct assignment

This further reduces the generated binary of vm_call_method from 566
bytes to 545 bytes on my machine, according to nm(1).

vm_insnhelper.c

@@ -3159,9 +3159,10 @@ vm_call_method(rb_execution_context_t *ec, rb_control_frame_t *cfp, struct rb_ca
 		else {
 		    /* caching method info to dummy cc */
 		    VM_ASSERT(vm_cc_cme(cc) != NULL);
+                    const struct rb_callcache cc_on_stack = *cc;
                     return vm_call_method_each_type(ec, cfp, calling, &(struct rb_call_data) {
                         .ci = ci,
-                        .cc = &VM_CC_ON_STACK(cc->klass, vm_cc_call(cc), { 0 }, vm_cc_cme(cc)),
+                        .cc = &cc_on_stack,
                     });
 		}
 	    }