arch: Make the decoder decode() method virtual.

Change-Id: I60f0c4ffbd63069caaee190a78f007df79b61808
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52078
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
7 files changed