arch: Make the decoder reset() method virtual.

This is called very infrequently, and so it's virtual overhead is
practically irrelevant.

Change-Id: If92cd96f75234c65c4cdffe392c32cfdd9b0c8cb
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52074
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

@@ -57,12 +57,6 @@ class Decoder : public InstDecoder
     {
     }
 
-    void
-    reset()
-    {
-        instDone = false;
-    }
-
     // Use this to give data to the predecoder. This should be used
     // when there is control flow.
     void