arch: introduce minimal required gcc version

arch: introduce minimal required gcc version

Some CPU variants require that a recent-enough gcc be selected. For
example, ARM's cortex-a35 requires gcc-5, while cortex-a73 requires
gcc-7. Same goes for other architectures, of course.

Currently, we hard-code every such conditions in the gcc version choice,
as well as in the individual external toolchains.

However, as we add even more CPU variants, the conditions are getting
more and more complex to write and maintain.

Introduce new symbols, that architectures can select if they have a
specific requirement on the gcc version. gcc and external toolchains
can then properly depend on those symbols.

The burden of maintaining the requirements on the gcc version now falls
down to the architeture, instead of being split up in gcc and all the
external toolchains.

As the oldest gcc version to handle, we can either choose gcc-4.9, as
the oldest version we support in our internal toolchain, or choose
gcc-4.8, as the oldest external toolchain we support (except for the
custom ones, but they'll be handled specifically in upcoming changes).
We choose to go back up to gcc-4.8.

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