python,scons: Move readCommand and compareVersions into site_scons.

These functions are only used by scons, so it makes sense to move them
to site_scons/gem5_scons/util.py.

Change-Id: If2b3995f208cb71adf3c59aac4eabe378c47f94f
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41599
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/systemc/core/SConscript

@@ -23,8 +23,6 @@
 # (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 # OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 
-from m5.util import compareVersions
-
 import gem5_scons
 
 Import('*')