Add support for ARC SMP targets

[abrodkin]

OpenOCD might be used to work nicely with SMP targets, see http://openocd.org/doc/html/Config-File-Guidelines.html#definecputargetsworkinginsmp.

We may just define SMP system in OpenOCD configuration:

target smp $_TARGETNAME1 $_TARGETNAME2 

And then reset & resume commands will be sent to all cores in the SMP.
Which means with proper openocd.cfg generic OpenOCD runner could be used in Zephyr for running SMP-aware applications on SMP HW.

The problem is this SMP-related functionality requires architecture-specific changes and so far it has not been implemented for ARC.

Just as a reference to how it all started for ARM Cortex-A: https://repo.or.cz/openocd.git/blobdiff/b778b36f29b74d6d571df85f984ae684672ea162..5578935eff66ed6db4a8f6b6957ab671dd36732b:/src/target/cortex_a.c

But there're some more changes after :)

[EvgeniiDidin]

Pull request, which solves this issue:
#25