systemc, scons: Add a mechanism to hook in the systemc tests.

systemc, scons: Add a mechanism to hook in the systemc tests.

This mechanism scans the systemc test directories as described in their
original distribution. It tells scons how to build each test
executable, and also how to build a json manifest file which
lists all the tests and some properties about them.

Change-Id: I8ebc748c1aed71f0bb76e04a2040f15abd2837d9
Reviewed-on: https://gem5-review.googlesource.com/10956
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>