cpu: Include debug flags regardless of whether the ISA is null.

cpu: Include debug flags regardless of whether the ISA is null.

Whether debug flags are available has no interaction with what the ISA
is.

Change-Id: I71d9204f948618831796e6c7a4c16bbebfb1a4fb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18428
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>