systemc: Implement some simple accessor functions in sc_main.cc.
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 <68c46a606457643eab92053c1c05574abb26f861@lowepower.com>
Maintainer: Gabe Black <5c8aa1266c03186393d9e7b3b4e3b14239af8201@google.com>

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