arc: fix arch_ API implementations on current CPU

All of these should be no-ops for the following reasons:

1. User threads cannot configure memory domains, only supervisor
   threads.
2. The scope of memory domains is user thread memory access,
   supervisor threads can access the entire memory map.

Hence it's never required to reprogram the MPU on the current CPU
when a memory domain API is called.

This does not address the issue #27785 if a user thread in the domain
is running on some other CPU.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>