use link register to determine return address when appropriate in getStackTrace

On PowerPC and ARM, we can't rely on the return address having already
been saved on the stack on entry to a thunk, so we must look for it in
the link register instead.

src/arm.cpp

@@ -1838,7 +1838,11 @@ class MyArchitecture: public Assembler::Architecture {
 
     return index;
   }
-  
+
+  virtual bool hasLinkRegister() {
+    return true;
+  }
+
   virtual unsigned stackAlignmentInWords() {
     return StackAlignmentInWords;
   }