gcc: select the appropriate BR2_TOOLCHAIN_GCC_AT_LEAST_* option

This commit wires up the gcc version dependency mechanism in the
internal toolchain backend by making the gcc version choice in the gcc
package Config.in.host select the appropriate
BR2_TOOLCHAIN_GCC_AT_LEAST_* option.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>