binutils: allow the selection of upstream binutils 2.29 for ARC

binutils: allow the selection of upstream binutils 2.29 for ARC

As of today quite a lot of ARC stuff got merged in upsream
such that we may finally use upstream realease for building
and even running Linux kernel and rootfs built with vanilla
binutils.

Essentially upstream release is still a bit behind our
GitHub-based releases but let's try to use vanilla version
and if we're lucky enough with just a small set of back-ported patches
we'll be able to use vanilla release(s) for everything in Buildroot.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
[Thomas: fixup the condition to select the special binutils site for ARC.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>