Microchip has a fork of avr-gcc 5.4.0 which they are maintaining under the name XC8.
The source code for it is available here: https://www.microchip.com/en-us/tools-resources/archives/mplab-ecosystem
This set of repositories makes the source code more easily accessible and browsable. GCC version history up to 5.4.0 has been maintained as well as possible.