Which blocks do these signals correspond to? If they are S-function blocks that you have written, you can put the printf commands in the corresponding TLC file. Beyond that, you might need to use advanced Simulink Coder concepts depending on what kind of target you are generating and whether the signals are outputs of specific blocks/subsystems. If you only need to do this for debugging, I would recommend just putting in the printf commands manually into the generating code.
Note: What you're asking for is not supported on a generic embedded system and therefore is not part of the core Simulink/Embedded Coder features. Note that an stdout is not even present on a lot of embedded systems.
