systemc: Add return code verification to verify.py.

systemc: Add return code verification to verify.py.

This just checks whether gem5 exited of its own accord with a
successful error code, or in other words that it didn't hang or crash.
More checking will need to be added to verify the output against the
golden reference.

Change-Id: I1ddef56aa73b5f700743830bd6212804531c484f
Reviewed-on: https://gem5-review.googlesource.com/12053
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>