binutils: update to the latest ARC version by default

Following commit 36555b4c ("ARC:
switch to RC1 of upcoming arc-2015.06 tools"), the binutils package
only contains the hash information for the 2015.06-rc1 ARC version of
binutils (which is in fact no hash). However, when an external
toolchain is used, and binutils is built for the target, it's still
the old 2014.11 binutils version that gets used in binutils.mk,
causing a build failure because there is no hash available for this
version.

This commit fixes that by using 2015.06-rc1 as the default binutils
version on ARC, which is used when no host-binutils has been built.

Fixes:

  http://autobuild.buildroot.org/results/8f7/8f772e6fccb4f918120a7bb814da2224432d1c09/



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