toolchain: add support of ARCv2 architecture

This change adds support of ARC ISAv2 processors in
OpenWRT toolchain.

In general gcc for ARC may compile code for both ISA versions
simultaneously but libgcc will be built only for default
architecture that's why it's necessary to specify --with-cpu
on gcc configuration.

As for uClibc we need to use different configurations for
different ARC ISAs.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Felix Fietkau <nbd@openwrt.org>
Cc: Jo-Philipp Wich <jow@openwrt.org>
Cc: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 48739