systemc: Update the systemc example to use m5.systemc for sc_main.

Change-Id: I431d3f2c18964bac1a3f19eacfffd49cd6e50fa2
Reviewed-on: https://gem5-review.googlesource.com/c/16569
Maintainer: Gabe Black <gabeblack@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

util: Add some examples for using systemc code within gem5.

These examples have comments inside them that explain what they do.
There's also a README file which explains how to use the examples
generally, and at a high level what each one does.

Change-Id: I223963dc1c190289986b2ee5705910dbcad4a4c9
Reviewed-on: https://gem5-review.googlesource.com/c/13376
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>