arc: Honor DESTDIR in custom Makefile

Installing native binutils in Buildroot faile, because `prefix` is /usr, and
proper installation path on host is achieved via usage of DESTDIR variable.
Homegrown ARC Makefile was ignoring this variable and therefour it was
trying to install files into /usr/extlib, when doing native binutils build.
This patch teaches our Makefile to honor DESTDIR.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>