This is an official Apple package of all the common GCC toolchain bits for OS X. XCode has always installed these alongside the GUI and OS X frameworks. Now people can just download a 200MB package instead of a 2-3GB package. It's not a cross compiling solution.

Cross compiling OS X on Linux has been possible, though it involves bring framework files over from a real OS X box. Not sure about the legality.

http://wiki.lazarus.freepascal.org/Cross_compiling_OSX_on_Linux covers the work needed for that particular Pascal IDE program. I didn't find it worth the hassle compared to just having a Mac Mini accessible over SSH.