Print the address of the sentinel object for debugging purposes.

This object is always allocated in the non-moving space, and could be
the one triggering a failing assertion in
art::gc::collector::ConcurrentCopying::ProcessMarkStackRef. Print it
as part of the information dumped when this assertions fails.

Note: This is a temporary change that should be reverted after the
corresponding bug is closed.

Test: art/test.py
Bug: 116087961
Change-Id: I678989b4ad921fefbacbfc42218e18f7302cb174
diff --git a/runtime/runtime.cc b/runtime/runtime.cc
index 34b84f5..36b43b1 100644
--- a/runtime/runtime.cc
+++ b/runtime/runtime.cc
@@ -1101,6 +1101,10 @@
   sentinel_ = GcRoot<mirror::Object>(sentinel);
 }
 
+GcRoot<mirror::Object> Runtime::GetSentinel() {
+  return sentinel_;
+}
+
 static inline void CreatePreAllocatedException(Thread* self,
                                                Runtime* runtime,
                                                GcRoot<mirror::Throwable>* exception,