Temporarily enable futex version of ReaderWriterMutex.

To get some numbers.
Also fix a bug for when self == NULL.

Change-Id: I93189e74df7e01a0efd02491ea49ed371738863c
diff --git a/src/mutex.cc b/src/mutex.cc
index a3aec41..47ad582 100644
--- a/src/mutex.cc
+++ b/src/mutex.cc
@@ -353,7 +353,8 @@
       android_atomic_dec(&num_pending_writers_);
     }
   } while(!done);
-  exclusive_owner_ = static_cast<uint64_t>(self->GetTid());
+  DCHECK_EQ(state_, -1);
+  exclusive_owner_ = SafeGetTid(self);
 #else
   CHECK_MUTEX_CALL(pthread_rwlock_wrlock, (&rwlock_));
 #endif
diff --git a/src/mutex.h b/src/mutex.h
index 1738870..f69eb32 100644
--- a/src/mutex.h
+++ b/src/mutex.h
@@ -28,7 +28,11 @@
 #include "macros.h"
 #include "thread.h"
 
+#if defined(__APPLE__)
 #define ART_USE_FUTEXES 0
+#else
+#define ART_USE_FUTEXES 1
+#endif
 
 // Currently Darwin doesn't support locks with timeouts.
 #if !defined(__APPLE__)