Modify Caml_state->backtrace_last_exn properly (#2191)

(cherry picked from commit 369a926)

Author: mshinwell

ocaml/otherlibs/systhreads/st_stubs.c

@@ -286,7 +286,8 @@ static void restore_runtime_state(caml_thread_t th)
   caml_set_local_arenas(Caml_state, th->local_arenas);
   Caml_state->backtrace_pos = th->backtrace_pos;
   Caml_state->backtrace_buffer = th->backtrace_buffer;
-  Caml_state->backtrace_last_exn = th->backtrace_last_exn;
+  caml_modify_generational_global_root
+    (&Caml_state->backtrace_last_exn, th->backtrace_last_exn);
 #ifndef NATIVE_CODE
   Caml_state->trap_sp_off = th->trap_sp_off;
   Caml_state->trap_barrier_off = th->trap_barrier_off;