gcc: mark 4.7.x as deprecated

We're already using 4.9.x as default, and have 4.8.x on the lower side
together with 5.x (5.3.0) on the higher side.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>