gcc: switch to gcc 4.8.x as the default compiler version

gcc 4.7.x is going to be retired soon, and now that gcc 4.9.0 is out,
it's time for Buildroot to switch to gcc 4.8.x as the default gcc
compiler version.

See https://gcc.gnu.org/ml/gcc/2014-05/msg00324.html

 for details about
gcc 4.7.x life cycle.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>