systemc: Implement some simple accessor functions in sc_main.cc.

These functions just read or write values with minimal amounts of
logic.

Change-Id: I22d5b49a2550a88a854d1619f08b0055c1312271
Reviewed-on: https://gem5-review.googlesource.com/10847
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>