fix ARM stack unwinding

We can't rely on the C++ compiler to save the return address in a
known location on entry to each function we might call from Java
(although GCC 4.5 seems to do so consistently, which is why I hadn't
realized the unwinding code was relying on that assumption), so we
must store it explicitly in MyThread::ip in each thunk.  For PowerPC
and x86, we continue saving it on the stack as always, since the
calling convention guarantees its location relative to the stack
pointer.

src/arm.cpp

@@ -2126,7 +2126,11 @@ class MyAssembler: public Assembler {
              &handlerConstant);
   }
 
-  virtual void saveFrame(unsigned stackOffset) {
+  virtual void saveFrame(unsigned stackOffset, unsigned ipOffset) {
+    Register link(LinkRegister);
+    Memory linkDst(ThreadRegister, ipOffset);
+    moveRM(&c, BytesPerWord, &link, BytesPerWord, &linkDst);
+
     Register stack(StackRegister);
     Memory stackDst(ThreadRegister, stackOffset);
     moveRM(&c, BytesPerWord, &stack, BytesPerWord, &stackDst);