mem: Make slicc generate some default methods explicitly.

mem: Make slicc generate some default methods explicitly.

Implicitly using the default copy constructor and assignment operator
is apparently deprecated, and gcc 9 will warn about it, breaking the
build.

Change-Id: Ida7a8a577e9d1cde9841eac7eee1af74563f1e27
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24927
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Bradford Beckmann <brad.beckmann@amd.com>
Reviewed-by: John Alsop <johnathan.alsop@amd.com>
Maintainer: Bradford Beckmann <brad.beckmann@amd.com>