Ok supose you use llvm to Cross-Platform development, There is no easy way to manage the gui stuff across multiple platforms using llvm. In the frontend language you need a library with a coherent wrapper for all cross platform gui stuff (CLang still doesnt have one) and in the backend target you need a "compiler backend" for every target platform. Supose for example you found a gui library that works on windows, linux (all flavors), osx, Android, Ios, WP7, Symbian, Webos, Blackberry with llvm, maybe SWT could help but the java frontend of llvm is incomplete, Maybe Qt Lighthouse could helps but the Qt targets for Ios and Android are still in beta .
Please correct me if I am wrong but today LLVM still doest offer a solution for the cross platform stuff.
Today Fasm could compile to arm for mobile http://arm.flatassembler.net/
but there is still a lot of work.