Some embedded boards use x86 CPU. Linux as Gentoo can be installed as for a regular PC. The only issue is speed and memory size required during compilation. It is possible to un pulg a harddisk (or sd card) from such boards and attach them via USB converter to a full blown PC and then chroot in the environment and do the compilation in this way. This works just without cross compilation if both computers are 32bit, since the big PC with its kernel can deal with binaries of the small PC.
An other way is using distcc. It works just without cross compilation if both PC's use the same compiler variant, a 686 C compiler will not produce binary code that runs on a small 486 device.