cpu: Use a unique_ptr to manage cached disassembly.

This is a fairly minor improvement, but avoids having to manually keep
track of that string pointer.

Change-Id: Ic3d4ddd9445920a110b36ab0cd64ff2289cf0139
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42967
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>

src/cpu/static_inst.hh

@@ -283,7 +283,7 @@ class StaticInst : public RefCounted, public StaticInstFlags
      * String representation of disassembly (lazily evaluated via
      * disassemble()).
      */
-    mutable std::string *cachedDisassembly = nullptr;
+    mutable std::unique_ptr<std::string> cachedDisassembly;
 
     /**
      * Internal function to generate disassembly string.
@@ -301,7 +301,7 @@ class StaticInst : public RefCounted, public StaticInstFlags
     {}
 
   public:
-    virtual ~StaticInst();
+    virtual ~StaticInst() {};
 
     virtual Fault execute(ExecContext *xc,
             Trace::InstRecord *traceData) const = 0;