arch,sim: Return whether or not a pseudo inst was recognized.

arch,sim: Return whether or not a pseudo inst was recognized.

Otherwise there's no way to distinguish whether return values are from
the calls themselves, including what they mean in the context (success
or failure?) or the pseudo inst dispatch function itself.

Change-Id: I3e71c277f175c69af0d1adeb3299d88d095dfa84
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25948
Tested-by: Gem5 Cloud Project GCB service account <345032938727@cloudbuild.gserviceaccount.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Alexandru Duțu <alexandru.dutu@amd.com>