You can place it in "Compiler settings -> Other options".
The assembly code will be in the *.o-files in this case (because C::B uses the -o-option), and it will be done for all processed files.
Or you can change the build-command for your file in the files properties (tab "advanced"):
check "Use custom command ..." and add
$compiler $options $includes -S -c $file
$compiler $options $includes -c $file -o $object
.
The example is for c++-files.
I double the command, so the file will be processed twice, one-time only the assembly is generated, the second time I do the normal processing, otherwise your build will be broken due to missing object-file or because an old object-file is taken (if it exists).