JNI direct buffer function speedup, part 2.

This converts the three direct buffer functions from JNI to internal VM
calls.  As a bonus, we grab PlatformAddress.osaddr directly instead of
retrieving it with PlatformAddress.toLong().

We're still calling through getEffectiveAddress(), which is where most
of the complexity lies.

Nudged a couple of comments.
diff --git a/vm/interp/Stack.c b/vm/interp/Stack.c
index d5c5fe9..6f418fd 100644
--- a/vm/interp/Stack.c
+++ b/vm/interp/Stack.c
@@ -528,6 +528,8 @@
  * from the varargs invocation where the C compiler does a widening
  * conversion when calling a function.  As a result, we have to be a
  * little more precise when pulling stuff out.
+ *
+ * "args" may be NULL if the method has no arguments.
  */
 void dvmCallMethodA(Thread* self, const Method* method, Object* obj,
     JValue* pResult, const jvalue* args)