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/riscv/decoder.cc

@@ -42,9 +42,7 @@ void Decoder::reset()
 {
     aligned = true;
     mid = false;
-    outOfBytes = true;
     emi = 0;
-    instDone = false;
 }
 
 void