stack guard - use __builtin_frame_address(0)

Pointers on the stack are pretty whack.
But clang builtins, they got yo' back.

Fixes: 258970199 (: or it'll get re-filed :)
Test: libbinderthreadstateutils_test

Change-Id: I4a7af39c680562a1b7a7c2c71c54eb371d857d5e

IPCThreadState.cpp

diff --git a/IPCThreadState.cpp b/IPCThreadState.cpp
index aa050fc..8631644 100644
--- a/IPCThreadState.cpp
+++ b/IPCThreadState.cpp
@@ -1147,7 +1147,7 @@
                 tr.offsets_size/sizeof(binder_size_t), freeBuffer, this);
 
             const void* origServingStackPointer = mServingStackPointer;
-            mServingStackPointer = &origServingStackPointer; // anything on the stack
+            mServingStackPointer = __builtin_frame_address(0);
 
             const pid_t origPid = mCallingPid;
             const char* origSid = mCallingSid;