We are building our own Atmel AVR8 GNU Toolchain for 64-bit Linux now because Microchip is bad about distributing binaries.
git clone https://github.com/profirmserv/pfs-avr8_toolchain_builder
cd pfs-avr8_toolchain_builder
vagrant up
vagrant ssh
/vagrant/avr8-toolchain-build.bash
When you get back from your smoke break, there should be a new tarball named something like avr8-gnu-toolchain-3.6.1.pfs-v1.linux.any.x86_64.tar.xz in the project directory.
You can also download binaries we've built.
We welcome contributions via GitHub pull request.