You don’t need to build your own cross-toolchain anymore - since version 8 of the toolchain, ARM and AArch64 sysroots are included in the zip (you can examine its contents), you just need to set it up using LINUX_MULTIARCH_ROOT variable and not deprecated LINUX_ROOT one. Edit your project’s DefaultEngine.ini and add (or change, if it exists) this section:
[/Script/LinuxTargetPlatform.LinuxTargetSettings]
TargetArchitecture=ArmUnknownLinuxGnueabihf
then re-build the Linux binaries (from scratch). You may encounter compiler errors since some targets will try to build ShaderCompileWorker first - please ignore ShaderCompileWorker ones as you don’t need SCW on the device.