ARC: Add EM as Opella target

This patch adds EM as a valid target for Opella XD. There is no separation
between EM4 and EM6, AUX registers controlling cache are always visible.
This doesn't cause any technical troubles, as unexisting registers are
zero-on-read/ignore-on-write.

To use this target *before* connecting to GDB server execute commands `set
arc opella-target arcem`.

Opella doesn't have XML file for EM. This file is added as
gdb/features/arc-opella-em.xml. It isn't used by GDB, it is for GDB server
only.

Signed-off-by: Anton Kolesov <Anton.Kolesov@synopsys.com>