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_arm.cc b/compiler/utils/arm/assembler_arm.cc
index dead8fd..e5f91dc 100644
--- a/compiler/utils/arm/assembler_arm.cc
+++ b/compiler/utils/arm/assembler_arm.cc
@@ -845,7 +845,7 @@
 
 void ArmAssembler::ExceptionPoll(ManagedRegister mscratch, size_t stack_adjust) {
   ArmManagedRegister scratch = mscratch.AsArm();
-  ArmExceptionSlowPath* slow = new ArmExceptionSlowPath(scratch, stack_adjust);
+  ArmExceptionSlowPath* slow = new (GetArena()) ArmExceptionSlowPath(scratch, stack_adjust);
   buffer_.EnqueueSlowPath(slow);
   LoadFromOffset(kLoadWord, scratch.AsCoreRegister(),
                  TR, Thread::ExceptionOffset<4>().Int32Value());