I took a pause a few years and don't know if this was addressed, but I see that the latest version of the avr8 GCC compiler suite by Microchip is using binutils v2.26 ; GCC v5.4.0 ; avrlibc from 20150208(? it seems the version.h date is not up to date)
I have always built my own tool suite, so I redid it and compiled binutils 2.31.1 ; GCC 8.2 ; and took the latest avr-libc from SVN (latest change from 2018-06-29, not much change anyway), it is very easy and build like I did years ago (I can put a recipe here if people are interested).
So why the suite by Microchip is 2+ years late ? Do they make modification in the code/lib? Is there some potential problem using a newer version of GCC?