webkitgtk24: depends on gcc >= 4.8

Webkit 2.4.x depends on gcc being >= 4.8.x so use the new
BR2_TOOLCHAIN_GCC_AT_LEAST_X_Y knob and drop the manual x86* external
toolchain exclusions.
Follow up in the midori package as well.

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