arm,x86,sim: Use the new return value suppression in GuestABI.

arm,x86,sim: Use the new return value suppression in GuestABI.

This gets rid of some dummy Return structure definitions. Also augment
the PseudoInst::pseudoInst dispatch function so it can store or not
store results, depending on what's needed at each call sight.

Change-Id: If4a53bc0a27e5214a26ef1a100c99948ca95418d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28289
Reviewed-by: Gabe Black <gabeblack@google.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>