travisci: Add support for ARC

Finally adding support for ARC boards in TravisCI.

To build for ARC boards we need to install Synopsys prebuilt toolchain
which we do here.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>