package/gcc: 4.8.x to 5.x: backport a fix for building gcc with gcc-6

When the host system compiler is gcc-6, this patch is needed to build
Buildroot toolchain and cleanly apply on gcc version 4.8.x to 5.x (not
needed for 4.7.x series).

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>