Change CStruct For Compatibility With Native Call JIT

Having the pointer to the actual data be first in
the struct lets us handle a CStruct exactly like
most other things in there.

src/6model/reprs/CStruct.h

@@ -19,13 +19,13 @@
  * C-friendly bit of memory and a copy to the GC-able, 6model objects in
  * sync. */
 struct MVMCStructBody {
-    /* GC-marked objects that our C structure points into. */
-    MVMObject **child_objs;
-
     /* Pointer to the actual C structure memory; we don't inline it
      * directly in the body, since it doesn't work so well if we get
      * something returned and are wrapping it. */
     void *cstruct;
+
+    /* GC-marked objects that our C structure points into. */
+    MVMObject **child_objs;
 };
 
 struct MVMCStruct {

src/core/nativecall.c

@@ -535,6 +535,10 @@ MVMJitGraph *MVM_nativecall_jit_graph_for_caller_code(
                 case MVM_NATIVECALL_ARG_UTF8STR:
                     if (is_rw) goto fail;
                     continue; /* already handled */
+                case MVM_NATIVECALL_ARG_CSTRUCT:
+                    if (is_rw) goto fail;
+                    arg_type = dst == -1 ? MVM_JIT_ARG_PTR : MVM_JIT_PARAM_PTR;
+                    break;
                 default:
                     goto fail;
             }