mem: Add sendAtomicBackdoor/recvAtomicBackdoor port methods.

mem: Add sendAtomicBackdoor/recvAtomicBackdoor port methods.

These both perform atomic accesses like their non-backdoor equivalents,
and also request a backdoor corresponding to the access.

The default implementation for recvAtomicBackdoor prints a warning
(once per port instance), calls recvAtomic to do the actual access,
and leaves the backdoor pointer as nullptr. That way if an object
doesn't know how to handle or transfer requests for a back door, it
automatically replies in a safe way that ignores the back door request.

Change-Id: Ia9fbbe9996eb4b71ea62214d203aa039a05f1618
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17590
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabeblack@google.com>