Add a new DB2 dialect that uses "cross join" for cross joins instead of ","

This issue is to track the fact that even though https://hibernate.atlassian.net/browse/HHH-11337 was fixed in Hibernate 5.1.4, the affected queries still fail on Sybase and DB2 (and possibly other databases) - see comment https://hibernate.atlassian.net/browse/HHH-11337?focusedCommentId=88106&page=com.atlassian.jira.plugin.system.issuetabpanels%3Acomment-tabpanel#comment-88106

This issue is specifically for fixing on DB2 by creating a new DB2 dialect.

This can be reproduced by running EntityJoinTest.testLeftOuterEntityJoinsWithImplicitInnerJoinInSelectClause against an affected database.