systemc: Increase the stack size for the sc_main fiber to 8MB.

This is a reasonable size for a stack, and the default size for a stack
on Linux as determined by some quick Googling. The sc_main fiber would
normally use the primary program stack if run under the standard
systemc implementation, and so might expect to have more room to play
with.

Change-Id: Ie12344939e7b249da203630ebc7dc773a387d716
Reviewed-on: https://gem5-review.googlesource.com/c/14396
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>

systemc: Seperate out the sc_main fiber and its bookkeeping.

By pulling out the sc_main fiber (scMainFiber), we can make it
available to different entities in the simulator and avoid having to
have parallel bookkeeping.

Also this will make it possible to hook into sc_main without putting
the code in sc_main.cc.

Change-Id: I7689441424238e9b2e4d2b48e945dea35fd8cc5d
Reviewed-on: https://gem5-review.googlesource.com/c/13977
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Gabe Black <gabeblack@google.com>