As you can see you aren't going to get much help in this forum on this type of question. There aren't too many places still developing code with Assembler and using DB2. I can't give you any answer as have not run across this issue as yet. I did look through SDSNMACS and while there are assembler macros, none of them use bigint, also, IBM has not updated in the application guide, they explain the data type conversion for almost every data type, but BIGINT. You should propbably hit IBM support for the answer. Good luck.
I just realized I misspoke. I was thinking that BIGINT was introduced in V8 and that is the manual I was looking at. When you look at the application programming and SQL guide for V9, it is there. In info Center search for BIGINT and its about 2/3 down in the resultset.