scons,systemc: In clang <= 6, -Wno-self-assign-overloaded doesn't exist.

In those versions of clang, use -Wno-self-assign.

Change-Id: Ic4f2fba6881f65fd40d0a4f3d3fb76574614b29a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/44385
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/systemc/core/SConscript

@@ -23,6 +23,10 @@
 # (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('*')
 
 if env['USE_SYSTEMC']:
@@ -63,6 +67,9 @@ if env['USE_SYSTEMC']:
         Source('python.cc')
         Source('sc_main_python.cc')
         append = {}
-        if env['CLANG']:
-            append['CCFLAGS'] = '-Wno-self-assign-overloaded'
+        with gem5_scons.Configure(main) as conf:
+            for flag in ('-Wno-self-assign-overloaded', '-Wno-self-assign'):
+                if conf.CheckCxxFlag(flag, autoadd=False):
+                    append['CCFLAGS'] = [flag]
+                    break
         Source('sc_time_python.cc', append=append)