save return address in arm.cpp's MyAssembler::saveFrame

This is necessary to allow safe stack unwinding (e.g. for exception
handling and garbage collection) from native code.

src/arm.cpp

@@ -1856,6 +1856,11 @@ class MyAssembler: public Assembler {
   }
 
   virtual void saveFrame(unsigned stackOffset, unsigned) {
+    Register returnAddress(LinkRegister);
+    Memory returnAddressDst
+      (StackRegister, arch_->returnAddressOffset() * BytesPerWord);
+    moveRM(&c, BytesPerWord, &returnAddress, BytesPerWord, &returnAddressDst);
+
     Register stack(StackRegister);
     Memory stackDst(ThreadRegister, stackOffset);
     moveRM(&c, BytesPerWord, &stack, BytesPerWord, &stackDst);