misc: Appease GCC 8

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>