mcu atmega328 only supported in assembler
This comes from the gcc compiler itself (which pre-dates the existence of a non-P 328), and I'm pretty sure that it's more complicated to fix than modifying a "config" file somewhere. (the IDE you update with a config file. Avrdude you update with a config file. Gcc you probably modify a source module somewhere and then recompile the compiler.)
Better to lie to everything and tell them you have a 328p.