Use ATTRIBUTE_UNUSED more.

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

Change-Id: If247dce58b72056f6eea84968e7196f0b5bef4da
diff --git a/runtime/base/allocator.cc b/runtime/base/allocator.cc
index 4f2fc07..f1d0a5f 100644
--- a/runtime/base/allocator.cc
+++ b/runtime/base/allocator.cc
@@ -49,15 +49,13 @@
   explicit NoopAllocator() {}
   ~NoopAllocator() {}
 
-  void* Alloc(size_t size) {
-    UNUSED(size);
+  void* Alloc(size_t size ATTRIBUTE_UNUSED) {
     LOG(FATAL) << "NoopAllocator::Alloc should not be called";
     UNREACHABLE();
   }
 
-  void Free(void* p) {
+  void Free(void* p ATTRIBUTE_UNUSED) {
     // Noop.
-    UNUSED(p);
   }
 
  private: