package/gcc: set e5500 and e6500 version mask

Freescale e5500 and e6500 cores are supported for versions >= 4.8
So filter out all of the older versions to avoid build failures.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>