Use ATTRIBUTE_UNUSED more.

Use it in lieu of UNUSED(), which had some incorrect uses.

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/base/arena_allocator.h b/runtime/base/arena_allocator.h
index 004895a..4e9282f 100644
--- a/runtime/base/arena_allocator.h
+++ b/runtime/base/arena_allocator.h
@@ -119,13 +119,13 @@
   ArenaAllocatorStatsImpl(const ArenaAllocatorStatsImpl& other) = default;
   ArenaAllocatorStatsImpl& operator = (const ArenaAllocatorStatsImpl& other) = delete;
 
-  void Copy(const ArenaAllocatorStatsImpl& other) { UNUSED(other); }
-  void RecordAlloc(size_t bytes, ArenaAllocKind kind) { UNUSED(bytes, kind); }
+  void Copy(const ArenaAllocatorStatsImpl& other ATTRIBUTE_UNUSED) {}
+  void RecordAlloc(size_t bytes ATTRIBUTE_UNUSED, ArenaAllocKind kind ATTRIBUTE_UNUSED) {}
   size_t NumAllocations() const { return 0u; }
   size_t BytesAllocated() const { return 0u; }
-  void Dump(std::ostream& os, const Arena* first, ssize_t lost_bytes_adjustment) const {
-    UNUSED(os); UNUSED(first); UNUSED(lost_bytes_adjustment);
-  }
+  void Dump(std::ostream& os ATTRIBUTE_UNUSED,
+            const Arena* first ATTRIBUTE_UNUSED,
+            ssize_t lost_bytes_adjustment ATTRIBUTE_UNUSED) const {}
 };
 
 template <bool kCount>