systemc: Implement sc_time::to_string.

systemc: Implement sc_time::to_string.

This is just another way to call sc_time::print, but it returns a
string instead of printing to a stream.

Change-Id: Idc90c539127e6153af9511bfe5f258b870362330
Reviewed-on: https://gem5-review.googlesource.com/c/12596
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>