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/arm64/assembler_arm64.cc b/compiler/utils/arm64/assembler_arm64.cc
index 0e17512..eb851f9 100644
--- a/compiler/utils/arm64/assembler_arm64.cc
+++ b/compiler/utils/arm64/assembler_arm64.cc
@@ -32,10 +32,8 @@
 #endif
 
 void Arm64Assembler::FinalizeCode() {
-  if (!exception_blocks_.empty()) {
-    for (size_t i = 0; i < exception_blocks_.size(); i++) {
-      EmitExceptionPoll(exception_blocks_.at(i));
-    }
+  for (Arm64Exception* exception : exception_blocks_) {
+    EmitExceptionPoll(exception);
   }
   ___ FinalizeCode();
 }