ARM: Fix subtle bug in LDM.

If the instruction faults mid-op the base register shouldn't be written back.

src/cpu/static_inst.hh

@@ -267,6 +267,7 @@ class StaticInstBase : public RefCounted
 
     void setLastMicroop() { flags[IsLastMicroop] = true; }
     void setDelayedCommit() { flags[IsDelayedCommit] = true; }
+    void setFlag(Flags f) { flags[f] = true; }
 
     /// Operation class.  Used to select appropriate function unit in issue.
     OpClass opClass()     const { return _opClass; }