MDEV-20525 rocksdb debug compilation fails on Windows due to unresolved my_assert variable

MYSQL_PLUGIN_IMPORT did not work correctly for the Rocksdb helper library
rocksdb_aux_lib, because that library was not compiled with
-DMYSQL_DYNAMIC_PLUGIN.

Fix dbug such that it does not depend on exported data, only on functions
(which do not need MYSQL_PLUGIN_IMPORT decoration)

Use a "getter" function _db_my_assert() instead of DLL-exported variable.

dbug/dbug.c

@@ -2258,6 +2258,11 @@ static int default_my_dbug_sanity(void)
   return 0;
 }
 
+extern my_bool my_assert;
+my_bool _db_my_assert(void)
+{
+  return my_assert;
+}
 #else
 
 /*

include/my_dbug.h

@@ -35,7 +35,6 @@ struct _db_stack_frame_ {
 };
 
 struct  _db_code_state_;
-extern MYSQL_PLUGIN_IMPORT my_bool my_assert;
 extern  my_bool _dbug_on_;
 extern  my_bool _db_keyword_(struct _db_code_state_ *, const char *, int);
 extern  int _db_explain_(struct _db_code_state_ *cs, char *buf, size_t len);
@@ -59,6 +58,7 @@ extern  void _db_dump_(uint _line_,const char *keyword,
 extern  void _db_end_(void);
 extern  void _db_lock_file_(void);
 extern  void _db_unlock_file_(void);
+extern  my_bool _db_my_assert(void);
 extern  FILE *_db_fp_(void);
 extern void _db_flush_(void);
 extern void dbug_swap_code_state(void **code_state_store);
@@ -105,7 +105,7 @@ extern int (*dbug_sanity)(void);
 #define DBUG_LOCK_FILE _db_lock_file_()
 #define DBUG_UNLOCK_FILE _db_unlock_file_()
 #define DBUG_ASSERT(A) do { if (!(A)) { _db_flush_();                      \
-      if (my_assert) assert(A);                                            \
+      if (_db_my_assert()) assert(A);                                            \
       else fprintf(stderr, "%s:%d: assert: %s\n", __FILE__, __LINE__, #A); \
 }} while (0)
 #define DBUG_SLOW_ASSERT(A) DBUG_ASSERT(A)

include/my_sys.h

@@ -266,7 +266,7 @@ extern ulong    my_sync_count;
 extern uint	mysys_usage_id;
 extern int32    my_file_opened;
 extern my_bool	my_init_done, my_thr_key_mysys_exists;
-extern MYSQL_PLUGIN_IMPORT my_bool my_assert;
+extern my_bool my_assert;
 extern my_bool  my_assert_on_error;
 extern myf      my_global_flags;        /* Set to MY_WME for more error messages */
 					/* Point to current my_message() */