systemc: Implement the sc_time_tuple class.

systemc: Implement the sc_time_tuple class.

This class is non-standard and is an implementation detail in
Accellera's implementation, but is referred to directly by the tests.
It does the same thing as the time printing function, so rather than
having duplicate code the printing function now uses the sc_time_tuple
class even though it was doing fine on its own already.

Change-Id: I69594ed0651f212ded6d979d60523bb3b0a789b1
Reviewed-on: https://gem5-review.googlesource.com/c/13189
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>