flambda-backend: Fix missing restore/save_runtime_state in caml_c_thread_register for runtime5 (#2739)

(cherry picked from commit c3984ff)

Author: mshinwell

otherlibs/systhreads/st_stubs.c

@@ -848,6 +848,11 @@ CAMLexport int caml_c_thread_register(void)
   }
   /* Associate the thread descriptor with the thread */
   st_tls_set(caml_thread_key, (void *) th);
+
+  /* Prepare Caml_state for allocation etc., so its various members correspond
+     to those for the newly-registered thread */
+  restore_runtime_state(th);
+
   /* Allocate the thread descriptor on the heap */
   th->descr = caml_thread_new_descriptor(Val_unit);  /* no closure */
 
@@ -856,6 +861,9 @@ CAMLexport int caml_c_thread_register(void)
     sync_check_error(err, "caml_register_c_thread");
   }
 
+  /* Save any modifications to Caml_state back to the thread descriptor */
+  save_runtime_state();
+
   /* Release the master lock */
   thread_lock_release(Dom_c_threads);
   return 1;