On a pi (which runs linux already) you shouldn't need a cross compiler. Downloading the Linux sources and building on the target machine should work (you would need to install all the dependencies from the repository for the pi first). It may take a long time to complete but I don't see why it wouldn't work (although also haven't done it 
). The developer blog has instructions for compiling from source. Its possible (I'm running on Windows not Linux) that the standard Linux install compiles to install (although it may also be a binary image which would be X86 specific). Trying a linux install may be the easy first step, and if that fails look to the sources. Easiest of all would be to check the repository for the PI and see if someone else has already ported Fritzing, if they have an install from the repository will be easiest of all.
Peter Van Epp