I do not believe a lexer is currently included for that language (see 
Languages supported by Code::Blocks lexers).  You could test if any of the other assembly languages Code::Blocks does support have close enough syntax to work.  If you have time, you could also 
create a new lexer to support AT&T Assembly.
Edit: I did a bit of reading, and it appears 
MASM Assembly *might* be what you are looking for (note that I am inexperienced with assembly, so I 
could be completely am wrong).