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

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>