Yup, force ignore dependencies ie pacman -Udf binutils-avr-2.20.1-1-x86_64.pkg.tar.xz let me try binutils-avr from dsh with the patches applied 
It works! I've compiled and uploaded the Blink sketch without any additional global var declared and the sketch runs fine now. So I guess I don't need the patched gcc-avr for the moment. The problem is narrowed down to the binutils package it seems.
(no luck of course finding patches for binutils-avr 2.21)