systemc: Use the new python interface to delay fixing the timescale.

systemc: Use the new python interface to delay fixing the timescale.

This is necessary if an sc_time object is constructed globally, either
directly or indirectly, before python is available to fix the
timescale. The call will be deferred until the interpretter is up and
ready.

Change-Id: I486c0a90d44a0e0f0ad8c530b7148e1cff04a5cc
Reviewed-on: https://gem5-review.googlesource.com/12070
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>