systemc: Extend the execute phase of verify.py.

systemc: Extend the execute phase of verify.py.

Add -j and --timeout options to the execute phase of verify.py.

The --timeout option is implemented using the timeout utility program
which is assumed to be available on the host system. Python 3.3 added
a timeout argument to the subprocess module which is an alternative
approach, but then we would be dependent on python 3.3.

-j is implemented using the standard multiprocess.pool.ThreadPool
class.

Change-Id: I15b92f2b14de6710e2027a6a19984b2644b2a8df
Reviewed-on: https://gem5-review.googlesource.com/12051
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>