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>

src/arch/power/decoder.hh

@@ -82,7 +82,7 @@ class Decoder : public InstDecoder
 
   public:
     StaticInstPtr
-    decode(PCStateBase &next_pc)
+    decode(PCStateBase &next_pc) override
     {
         if (!instDone)
             return NULL;