toolchain-external: support gcc < 4.3

We currently support gcc as old as 4.3. However, Buildroot works
perfectly well with even older gcc versions (tested with 4.1). So we
can add an option BR2_TOOLCHAIN_EXTERNAL_GCC_OLD to support that. The
help text of this option is written with plenty of discouragement.

We use _OLD and not something like _PRE_4_3, because at some point we
will likely remove the 4.3 option and what would then require a name
change.

We don't set any _AT_LEAST option in this case because it's no use -
there is no lower bound on the version in this case. We therefore leave
BR2_TOOLCHAIN_GCC_AT_LEAST empty (the implicit default). When it is
empty, we don't do a version check at all in check_gcc_version
(previously we errored out when it was empty).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>