systemc: Make sc_main run in its own fiber.

systemc: Make sc_main run in its own fiber.

The fiber will run until either sc_main returns, or until sc_start is
called. If sc_start is called, then the fiber will only be paused and
waiting for simulation cycles to be run by gem5. Once sc_pause and
sc_stop are implemented, if those are called the sc_main fiber will
be re-entered and allowed to run further towards completion.

Change-Id: I4df94f4f6fed8d49471732619a203d734d9a13a6
Reviewed-on: https://gem5-review.googlesource.com/10849
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>