The process of creating a cross compiler or more accurately, cross toolchain, is not trivial. If you do not already have one, I suggest using something such as Crosstool-NG
and others, to aid you. You will need to understand some key things about your target environment, such as the hardware architecture (CPU and CPU variant type, and perhaps other CPU factors such as MMU and FPU), OS if present, standard C library, and some build-host based factors. It is not quite as simple as a complete mix & match of toolchain components, as the cross dependencies are abundant. If building a cross toolchain, be prepared to spend a considerable time trying different combinations before you find one that works for your build-host and target. Don't even think about trying to build one yourself.