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. Because this file uses an obsolete XML
format for target description it is undesirable to add it to the repository,
so it available as Gist: https://gist.github.com/anthony-kolesov/7193146.
Ultimately Ashling Gdbservser should be updated to use new XML target
descriptions and the need for this file will disappear.

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