systemc: Add a non-standard sc_time constructor and from_string.

The sc_time constructor was being called, but because of implicit type
conversions, a const char * was being treated as a bool and totally
unrelated constructor was being called.

This change adds and implements the missing but non-standard
constructor. It also implements the from_string function which uses
that constructor.

Change-Id: I21e7e40fd1a8d1c579b1abdc2036d016501f510c
Reviewed-on: https://gem5-review.googlesource.com/c/13191
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Move some constants out of sc_time.cc for other files to use.

Change-Id: Ic88c6834dfe980022e58a3d859ea53193a55bbb1
Reviewed-on: https://gem5-review.googlesource.com/c/12971
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>