Do not make copies of nSIM.

Setup environment variables for nSIM OSCI manually so that there is no need in making private copy of nSIM with updated systemc/scripts/config.sh.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>
Build em-extended toolchain in respective directory, not `arc`.
Support casual nSIM OSCI kernel image.