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/generic/decoder.hh

@@ -55,6 +55,12 @@ class InstDecoder
 
     virtual StaticInstPtr fetchRomMicroop(
             MicroPC micropc, StaticInstPtr curMacroop);
+    virtual void
+    reset()
+    {
+        instDone = false;
+        outOfBytes = true;
+    }
 
     void *moreBytesPtr() const { return _moreBytesPtr; }
     size_t moreBytesSize() const { return _moreBytesSize; }