Pass self to lock methods.

This avoids frequent recomputation of
Thread::Current/pthread_getspecific.

Also add a futex based reader/writer mutex that is disabled.

Change-Id: I118fdb99ef1d1c4bfda6446ba3a0d8b6ab31eaee
diff --git a/src/runtime_linux.cc b/src/runtime_linux.cc
index e503358..85eeb8f 100644
--- a/src/runtime_linux.cc
+++ b/src/runtime_linux.cc
@@ -21,6 +21,7 @@
 #include <sys/utsname.h>
 
 #include "logging.h"
+#include "mutex.h"
 #include "stringprintf.h"
 #include "utils.h"