Move Assemblers to the Arena.

And clean up some APIs to return std::unique_ptr<> instead
of raw pointers that don't communicate ownership.

Change-Id: I3017302307a0253d661240750298802fb0d9585e
diff --git a/compiler/utils/arm/assembler_arm32.h b/compiler/utils/arm/assembler_arm32.h
index e3e05ca..bc6020e 100644
--- a/compiler/utils/arm/assembler_arm32.h
+++ b/compiler/utils/arm/assembler_arm32.h
@@ -30,8 +30,7 @@
 
 class Arm32Assembler FINAL : public ArmAssembler {
  public:
-  Arm32Assembler() {
-  }
+  explicit Arm32Assembler(ArenaAllocator* arena) : ArmAssembler(arena) {}
   virtual ~Arm32Assembler() {}
 
   bool IsThumb() const OVERRIDE {