arch,cpu: Make branching() a virtual member of PCStateBase.

Change-Id: I4422d07024e97dbd67e97ad95a16e1b06fd6be12
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/52066
Tested-by: kokoro <noreply+kokoro@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/cpu/simple/base.cc

@@ -461,8 +461,7 @@ BaseSimpleCPU::advancePC(const Fault &fault)
     SimpleExecContext &t_info = *threadInfo[curThread];
     SimpleThread* thread = t_info.thread;
 
-    const bool branching =
-        thread->pcState().as<TheISA::PCState>().branching();
+    const bool branching = thread->pcState().branching();
 
     //Since we're moving to a new pc, zero out the offset
     t_info.fetchOffset = 0;