avr-mcus.def currenly lists at43usb320 as belonging to avr31. This is not correct because the device can only handle 64 KiB of (external) program memory but avr31 is for devices with 128 KiB (supports ELPM which is not supported by at43usb320). As the data sheet does not mention the exact instruction set supported, it's unclear if avr3 is the correct multilib (likely) or avr35 (if MOVW) or even avr5 (of MUL).