systemc: Move systemc disabling checks to SConsopts.

This will ensure that the value of USE_SYSTEMC is consistent throughout
the build. It also has the side effect that USE_SYSTEMC can be forced
to a particular value if you're confident you know what you're doing
and want to override these checks.

Change-Id: I0f2d1153245ff17ce4a828c6b7496cb9ded6bd5b
Reviewed-on: https://gem5-review.googlesource.com/c/16810
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Enable systemc support by default.

Change-Id: I9375518a54e14413a499d065f5bc5e1031834c81
Reviewed-on: https://gem5-review.googlesource.com/c/13535
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>

systemc: Add a stubbed out sc_object class.

Also add a SConsopt variable USE_SYSTEMC to hide systemc support until
it's usable.

Change-Id: Ibb37483432b147ee690a36bb5c8dd74f1c4c7ae4
Reviewed-on: https://gem5-review.googlesource.com/10821
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>