Makefile: Make sure gcc is built before system.img in developer-* builds

This is a workaround for make 3.81 not seeing the dependency introduced
in native-toolchain/Android.mk.

Change-Id: Ie64c39c4904f6ca45978802a6494f55c667add84
Signed-off-by: Bernhard Rosenkränzer <Bernhard.Rosenkranzer@linaro.org>