toolchain/external-custom: hide versions too old for the current arch

toolchain/external-custom: hide versions too old for the current arch

When an architecture expresses a requirement on the gcc version, limit
the version choice in the custom external toolchain.

The rationale being that there is no point in offering that version to
the user if we know before-hand that the gcc version will not work for
that architecture.

All versions below the minimum we support is just made conditional to
that minimum as well, including the "older" entry.

However, this means that the "older" entry is no longer available when
the architecture requires a minimum gcc version. A user who wants to use
a toolchain with a gcc older than the minimum will have no choice but to
realise the toolchain is not suitable (or lie and we would catch that
when checking the gcc version anyway).

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>