i#1966 ARM/AArch64 testing: Fix Travis builds broken by 0116719.
i#1966 ARM/AArch64 testing: Fix Travis builds broken by 0116719.

It turns out that g++-multilib is incompatible with the cross-compiler
packages, so add a job to the matrix with the environment variable CROSS.

Review-URL: https://codereview.appspot.com/307010043

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio