Sorry for the long time that i didn't write something here.
Since a while, Arduino changed the AVR compiler to 4.9.2. With this version it is possible to compile now for the new chip without patching it. I wrote now a package which do exactly this.
This package contains also the specific Atmel files for this MCU. Because the header file io.h doesn't know about this chip, I used a small hook to get it known. Another hook is to let the linker know about this chip. You can find this in the platform.txt.
You can install the package just copy this URL into your Arduino IDE:
Check out my repository: