Many to Many with BaseClass and tablePerHierarchy false

Based on the [official example](http://docs.grails.org/3.0.17/ref/Database%20Mapping/joinTable.html) the following example will fail with

`org.hibernate.AssertionFailure: Table mm_author_books not found
`

As the the similar unidirectional many-to-many mapping works just as expected with even the potentially identical mapping table I assume it's a bug.

UPDATE: if you use `tablePerConcreteClass true` instead of `tablePerHierarchy false`
it works. As we dont use an abstract base class both will create the same DB-mapping.

```
class SomeBase {

static mapping = {
tablePerHierarchy false

}
}

class Book extends SomeBase {

String title

static belongsTo = Author

static hasMany = [authors: Author]

static mapping = {
authors joinTable: [name: "mm_author_books", key: 'mm_book_id' ]
}
}

class Author extends SomeBase {

String name

List books

static hasMany = [books: Book]

static mapping = {
books joinTable: [name: "mm_author_books", key: 'mm_author_id']
}
}
```

Start with the SomeBase, Book, and Author mappings shown in the issue and reproduce the failure using tablePerHierarchy false. Compare the generated join-table behavior with tablePerConcreteClass true; done means the mm_author_books table is found and the many-to-many mapping works as in the unidirectional case.

Written by the indexing model from the issue text.