Add a heap HWM to the Thread*, for compiled code.

Also fix a bug in thread detach, and implement the thread exit callback.

Destroy our pthread_mutex_t instances, and check for success. (This will
catch us deleting locked Mutex instances.)

Change-Id: I26cf8117b825234f6c790e0cf70b2c025a743f84
diff --git a/src/runtime.h b/src/runtime.h
index 96513e6..99caa29 100644
--- a/src/runtime.h
+++ b/src/runtime.h
@@ -96,8 +96,8 @@
 
   ~Runtime();
 
-  size_t GetStackSize() const {
-    return stack_size_;
+  size_t GetDefaultStackSize() const {
+    return default_stack_size_;
   }
 
   ClassLinker* GetClassLinker() const {
@@ -130,7 +130,7 @@
   void RegisterRuntimeNativeMethods(JNIEnv*);
 
   // The default stack size for managed threads created by the runtime.
-  size_t stack_size_;
+  size_t default_stack_size_;
 
   ThreadList* thread_list_;