cpu: Fix SMT scheduling issue with the O3 cpu

The o3 cpu could attempt to schedule inactive threads under round-robin SMT
mode.

This is because it maintained an independent priority list of threads from the
active thread list.  This priority list could be come stale once threads were
inactive, leading to the cpu trying to fetch/commit from inactive threads.


Additionally the fetch queue is now forcibly flushed of instrctuctions
from the de-scheduled thread.

Relevant output:

24557000: system.cpu: [tid:1]: Calling deactivate thread.
24557000: system.cpu: [tid:1]: Removing from active threads list

24557500: system.cpu:
FullO3CPU: Ticking main, FullO3CPU.
24557500: system.cpu.fetch: Running stage.
24557500: system.cpu.fetch: Attempting to fetch from [tid:1]

src/cpu/o3/commit.hh

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2010-2012 ARM Limited
+ * Copyright (c) 2010-2012, 2014 ARM Limited
  * All rights reserved.
  *
  * The license below extends only to copyright in the software and shall
@@ -218,6 +218,9 @@ class DefaultCommit
     /** Takes over from another CPU's thread. */
     void takeOverFrom();
 
+    /** Deschedules a thread from scheduling */
+    void deactivateThread(ThreadID tid);
+
     /** Ticks the commit stage, which tries to commit instructions. */
     void tick();