ARM64: Update code after the VIXL 1.6 release.

We now leave the assembler buffer management to VIXL.

Change-Id: Ieefe83cf5cf5e1ab8c924b0e7dc03af6a55053ae
diff --git a/compiler/utils/arm64/assembler_arm64.cc b/compiler/utils/arm64/assembler_arm64.cc
index c82b4f0..25e02c3 100644
--- a/compiler/utils/arm64/assembler_arm64.cc
+++ b/compiler/utils/arm64/assembler_arm64.cc
@@ -42,12 +42,12 @@
 }
 
 size_t Arm64Assembler::CodeSize() const {
-  return ___ SizeOfCodeGenerated();
+  return vixl_masm_->BufferCapacity() - vixl_masm_->RemainingBufferSpace();
 }
 
 void Arm64Assembler::FinalizeInstructions(const MemoryRegion& region) {
   // Copy the instructions from the buffer.
-  MemoryRegion from(reinterpret_cast<void*>(vixl_buf_), CodeSize());
+  MemoryRegion from(vixl_masm_->GetStartAddress<void*>(), CodeSize());
   region.CopyFrom(0, from);
 }