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 <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu,gem5/gem5,gem5/gem5,TUD-OS/gem5-dtu