toolchain: add common gcc version hidden config options

This commit adds a number of hidden Config.in options, that will be
used to handle dependencies on the gcc version. We mimic the model
that was used for the kernel headers dependency mechanism.

These hidden options will be selected by the internal and external
toolchain backend logic respectively, in follow-up commits.

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