Abstract out fetching of break_address in debug mode

src/arm64/assembler-arm64.h

Index: src/arm64/assembler-arm64.h
diff --git a/src/arm64/assembler-arm64.h b/src/arm64/assembler-arm64.h
index a2ab4cd09f88a3971fae5cadf739f531749ea261..fd52688ff0b5c5371fe9f76a20736a6df2e1b414 100644
--- a/src/arm64/assembler-arm64.h
+++ b/src/arm64/assembler-arm64.h
@@ -887,6 +887,9 @@ class Assembler : public AssemblerBase {
   // instruction stream that call will return from.
   inline static Address return_address_from_call_start(Address pc);
 
+  // Return the code target address of the patch debug break slot
+  inline static Address break_address_from_return_address(Address pc);
+
   // This sets the branch destination (which is in the constant pool on ARM).
   // This is for calls and branches within generated code.
   inline static void deserialization_set_special_target_at(