At the moment there is no button to do it automatically. I have created a ticket, and will try to implement it, this should be quite straight forward
https://sourceforge.net/p/codeblocks/tickets/1261/But you can set it before debugging in Settings->Debugger->GDB/CDB->Default->Choose disassembly flavor
While debugging you can do it manually by:
1) closing the disassembly window
2) Selecting the "Debugger" tab in the bottom "Logs & others" window pane
3) Type
set disassembly-flavor att
for AT&T or
set disassembly-flavor intel
for intel in the "command" field and hit enter
4) Reopen the disassembly window