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/base/bitunion.hh

@@ -435,7 +435,7 @@ namespace BitfieldBackend
     inline std::ostream &
     bitfieldBackendPrinter(std::ostream &os, const char &t)
     {
-        os << (const int)t;
+        os << (int)t;
         return os;
     }
 
@@ -443,7 +443,7 @@ namespace BitfieldBackend
     inline std::ostream &
     bitfieldBackendPrinter(std::ostream &os, const unsigned char &t)
     {
-        os << (const unsigned int)t;
+        os << (unsigned int)t;
         return os;
     }
 }