RFA: Fix testsuite/91065

[Joern Wolfgang Rennecke]

Regression tested running plugin.exp testing gcc configured with 
--enable-checking=all (failure fixed),
--enable-checking=yes, and without --enable-checking (both no change).
2019-07-02  Joern Rennecke  <joern.renne...@riscy-ip.com>
        PR testsuite/91065
        * testsuite/gcc.dg/plugin/start_unit_plugin.c: Register a root tab
        to reference fake_var.

Index: /data/gcc-10-exp/gcc/gcc/testsuite/gcc.dg/plugin/start_unit_plugin.c
===================================================================
--- /data/gcc-10-exp/gcc/gcc/testsuite/gcc.dg/plugin/start_unit_plugin.c        
(revision 272931)
+++ /data/gcc-10-exp/gcc/gcc/testsuite/gcc.dg/plugin/start_unit_plugin.c        
(working copy)
@@ -40,6 +40,19 @@ gate_start_unit (void)
 
 static void start_unit_callback (void *gcc_data, void *user_data)
 {
+  static const struct ggc_root_tab root[] = {
+    {
+      &fake_var,
+      1,
+      sizeof (fake_var),
+      &gt_ggc_mx_tree_node,
+      &gt_pch_nx_tree_node
+    },
+    LAST_GGC_ROOT_TAB
+  };
+
+  register_callback ("start_unit", PLUGIN_REGISTER_GGC_ROOTS, NULL,
+                    (void *)root);
   if (integer_type_node) {
     fake_var = build_decl (UNKNOWN_LOCATION, VAR_DECL, 
                            get_identifier ("_fake_var_"),