Hi ChesLans,
There is no issue to build ROS2 for aarch64. You can follow the wiki instruction on your aarch64 machine and build the complete stack, it will just take some time =)
For cross-compilation, you can build a subset of ROS2 by following the instruction here.
@jcadam and @esteve already have successfully cross-compile the complete ROS2 for raspi (arm 32) and the Jetson TX2 (arm 64).