MDEV-7499 - System variables have broken default values on big endian

INFORMATION_SCHEMA.SYSTEM_VARIABLES.DEFAULT_VALUE had broken values on
big endian.

Default value is internally stored as longlong, while I_S references it's
pointer (longlong *) according to variable type (e.g. int, my_bool, etc). This
works well on little endian, but on big endian we always get 0 for such
variables.

sql/sql_class.h

@@ -2731,10 +2731,13 @@ class THD :public Statement,
   union
   {
     my_bool   my_bool_value;
+    int       int_value;
+    uint      uint_value;
     long      long_value;
     ulong     ulong_value;
     ulonglong ulonglong_value;
     double    double_value;
+    void      *ptr_value;
   } sys_var_tmp;
 
   struct {

sql/sql_plugin.cc

@@ -3253,7 +3253,31 @@ sys_var_pluginvar::sys_var_pluginvar(sys_var_chain *chain, const char *name_arg,
 uchar* sys_var_pluginvar::real_value_ptr(THD *thd, enum_var_type type)
 {
   if (type == OPT_DEFAULT)
-    return (uchar*)&option.def_value;
+  {
+    switch (plugin_var->flags & PLUGIN_VAR_TYPEMASK) {
+    case PLUGIN_VAR_BOOL:
+      thd->sys_var_tmp.my_bool_value= option.def_value;
+      return (uchar*) &thd->sys_var_tmp.my_bool_value;
+    case PLUGIN_VAR_INT:
+      thd->sys_var_tmp.int_value= option.def_value;
+      return (uchar*) &thd->sys_var_tmp.int_value;
+    case PLUGIN_VAR_LONG:
+    case PLUGIN_VAR_ENUM:
+      thd->sys_var_tmp.long_value= option.def_value;
+      return (uchar*) &thd->sys_var_tmp.long_value;
+    case PLUGIN_VAR_LONGLONG:
+    case PLUGIN_VAR_SET:
+      return (uchar*) &option.def_value;
+    case PLUGIN_VAR_STR:
+      thd->sys_var_tmp.ptr_value= (void*) option.def_value;
+      return (uchar*) &thd->sys_var_tmp.ptr_value;
+    case PLUGIN_VAR_DOUBLE:
+      thd->sys_var_tmp.double_value= getopt_ulonglong2double(option.def_value);
+      return (uchar*) &thd->sys_var_tmp.double_value;
+    default:
+      DBUG_ASSERT(0);
+    }
+  }
 
   DBUG_ASSERT(thd || (type == OPT_GLOBAL));
   if (plugin_var->flags & PLUGIN_VAR_THDLOCAL)

sql/sys_vars.h

@@ -219,6 +219,7 @@ class Sys_var_integer: public sys_var
     return scope() == SESSION ? (T*)(((uchar*)&max_system_variables) + offset)
                               : 0;
   }
+  uchar *default_value_ptr(THD *thd) { return (uchar*) &option.def_value; }
 };
 
 typedef Sys_var_integer<int, GET_INT, SHOW_SINT> Sys_var_int;
@@ -229,6 +230,31 @@ typedef Sys_var_integer<ulonglong, GET_ULL, SHOW_ULONGLONG> Sys_var_ulonglong;
 typedef Sys_var_integer<long, GET_LONG, SHOW_SLONG> Sys_var_long;
 
 
+template<> uchar *Sys_var_int::default_value_ptr(THD *thd)
+{
+  thd->sys_var_tmp.int_value= option.def_value;
+  return (uchar*) &thd->sys_var_tmp.int_value;
+}
+
+template<> uchar *Sys_var_uint::default_value_ptr(THD *thd)
+{
+  thd->sys_var_tmp.uint_value= option.def_value;
+  return (uchar*) &thd->sys_var_tmp.uint_value;
+}
+
+template<> uchar *Sys_var_long::default_value_ptr(THD *thd)
+{
+  thd->sys_var_tmp.long_value= option.def_value;
+  return (uchar*) &thd->sys_var_tmp.long_value;
+}
+
+template<> uchar *Sys_var_ulong::default_value_ptr(THD *thd)
+{
+  thd->sys_var_tmp.ulong_value= option.def_value;
+  return (uchar*) &thd->sys_var_tmp.ulong_value;
+}
+
+
 /**
   Helper class for variables that take values from a TYPELIB
 */
@@ -386,6 +412,11 @@ class Sys_var_mybool: public Sys_var_typelib
   { var->save_result.ulonglong_value= (ulonglong)*(my_bool *)global_value_ptr(thd, 0); }
   void global_save_default(THD *thd, set_var *var)
   { var->save_result.ulonglong_value= option.def_value; }
+  uchar *default_value_ptr(THD *thd)
+  {
+    thd->sys_var_tmp.my_bool_value= option.def_value;
+    return (uchar*) &thd->sys_var_tmp.my_bool_value;
+  }
 };
 
 /**