error linking lto1 for target avr

[Steven Bosscher]

> In avr.c there is:
>
> ...
> #include "c-family/c-common.h"
> ...

That is the problem: avr.c should be language independent. Here you
are trying to link code calling C-family functions in a non-C language
(lto1 is just another front end for gcc, just like
cc1/cc1plus/etc...).

You should use extra target files for language specific target code.
Plenty examples in config/*/*-c.* ...

Ciao!
Steven