binutils: fix target binutils on ARC

When an external toolchain is used on ARC, BR2_BINUTILS_VERSION is
undefined, so we use binutils 2.22, which doesn't support ARC and
anyway isn't available from the ARC download location.

So, let's defined a default ARC binutils version in binutils.mk.

Fixes:

  http://autobuild.buildroot.org/results/52a/52abadacd7aab2d5c11d094937f28198bf220662/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>