misc: Appease GCC 8

GCC 8 adds a number of new warnings to -Wall which generate errors.

- Fix memset to 0 for structs by adding casts.
- Fix cast with const when the const was ignored.
- Fix catch a polymorphic type by value

We now compile with GCC 8!

Change-Id: Iab70ce11190eee67608fc25c0bedff170152b153
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/11949
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/cpu/testers/traffic_gen/pygen.cc

@@ -74,7 +74,7 @@ PyTrafficGen::nextGenerator()
             metaGenerator->cast<std::shared_ptr<BaseGen>>();
         metaGenerator++;
         return gen;
-    } catch (py::cast_error) {
+    } catch (py::cast_error&) {
         fatal("Meta generator didn't return a valid trace generator\n");
     }
 }