arch,base,cpu: Add some default constructors/operators explicitly.

Having them implicitly is apparently deprecated and throws a warning
in gcc 9, breaking the build.

Change-Id: Id4e3074966d1ffc6dd1aed9397de5eea84400027
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/24926
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>
5 files changed