Abstract out fetching of break_address in debug mode

src/x87/assembler-x87-inl.h

Index: src/x87/assembler-x87-inl.h
diff --git a/src/x87/assembler-x87-inl.h b/src/x87/assembler-x87-inl.h
index 07a8751099c1c6844397332b30501f7d216a806c..98e490f7afaf85f5eb14e83f75cfc7d28e75e454 100644
--- a/src/x87/assembler-x87-inl.h
+++ b/src/x87/assembler-x87-inl.h
@@ -483,6 +483,11 @@ Address Assembler::target_address_from_return_address(Address pc) {
 }
 
 
+Address Assembler::break_address_from_break_address(Address pc) {
+  return pc - Assembler::kPatchDebugBreakSlotReturnOffset;
+}
+
+
 Displacement Assembler::disp_at(Label* L) {
   return Displacement(long_at(L->pos()));
 }