toolchain-external: add Synopsys DesignWare tools for ARC cores

With this change we add pre-built external toolachins for DesignWare ARC
cores. All currently existed flavours are supported:
 * ARC 700 and ARC HS cores
 * Little- and big-endian configurations

These pre-built tools are built with build scripts available here
(https://github.com/foss-for-synopsys-dwc-arc-processors/toolchain/tree/arc-2014.12

)
and correspond to arc-2014.12 release of sources.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Anton Kolesov <akolesov@synopsys.com>
Cc: "Yann E. MORIN" <yann.morin.1998@free.fr>