package/gcc: remove gcc 4.8

We're already using 5.x as default, and have 4.9.x on the lower side
together with 7.x (7.1) on the higher side.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
CC: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>