By default, these libraries are located at $ROS_HOME/eros/toolchains and $ROS_HOME/eros/platforms (i.e. if you are on linux, ~/.ros/eros/toolchains). However, they can be redirected when calling rostoolchain or rosplatform by using the --dir option.
They can be more permanently redirected by editing the eros configuration file ($ROS_HOME/eros/eros.cfg).
If you need to modify these, you can freely modify them by hand without disturbing anything.
Feel free to contact the developers if you wish to add your custom user-defined modules to the official eros_toolchains or eros_platforms library. The same also applies if you think any pre-defined configurations need modification.
Probably the best place to do so is via the eros issue tracker.