The latest developer package uploads into Ubuntu 14.04 "Trusty Tahr" allow for cross-compiling CMake-based projects, e.g. compiling Mir and Unity 8 for ARM from an x86 system.
According to Dimitri John Ledkov, this cross-compile CMake support in Ubuntu 14.04 works for Mir, Unity 8, Ubuntu System Settings, Unity Notifications, and a wide range of packages written against the CMake build system.
After an initial setup, it's very easy to compile projects to different targets. More details can be found on ubuntu-devel