Valgrind cleanliness, part 6: Introduce OS::TearDown, cleaning up 2 mutexes.

src/platform.h

Index: src/platform.h
diff --git a/src/platform.h b/src/platform.h
index 3b2aa3c8bafdf7c2a11ce1190769ef9eeac168b6..168791a0a4a881df549f1dc20830a2c006c220b3 100644
--- a/src/platform.h
+++ b/src/platform.h
@@ -123,6 +123,9 @@ class OS {
   // called after CPU initialization.
   static void PostSetUp();
 
+  // Clean up platform-OS-related things. Called once at VM shutdown.
+  static void TearDown();
+
   // Returns the accumulated user time for thread. This routine
   // can be used for profiling. The implementation should
   // strive for high-precision timer resolution, preferable