systemc: Implement some of the basics of sc_time.

systemc: Implement some of the basics of sc_time.

This is a very incomplete implementation, but is enough to unblock
implementing some other dependent features.

Change-Id: Ibd2f3476fe01389e277c6956d48337f551d14acd
Reviewed-on: https://gem5-review.googlesource.com/12034
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>