ARC: switch to RC1 of upcoming arc-2015.06 tools

Even though this is only RC1 it's been heavily used internally so it should
not be any worse than existing arc-2014.12.

Moreover this relase (and so its RC1) finally delivers support of NPTL
for ARC in uClibc.

That's why it would be good to allow interested users to start trying it
(for example WebKit and apps that use WebKit could be successfully built
and run) also it will be helpful to run that new toolchain through
autobuilder in attempt to find any hidden regressions so we have a solid
toolchain for release.

If there's an interest in that patch more patches will follow with
subsequent RCs and essentially on appearence or relese Buildroot will be
updated with it.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: arc-buildroot@synopsys.com
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>