Tidy logging code not using UNIMPLEMENTED.

Change-Id: I7a79c1671a6ff8b2040887133b3e0925ef9a3cfe
diff --git a/runtime/stack_map.h b/runtime/stack_map.h
index b1c46a9..a58ecab 100644
--- a/runtime/stack_map.h
+++ b/runtime/stack_map.h
@@ -286,7 +286,7 @@
       }
     }
     LOG(FATAL) << "Unreachable";
-    return StackMap(MemoryRegion());
+    UNREACHABLE();
   }
 
   StackMap GetStackMapForNativePcOffset(uint32_t native_pc_offset) {
@@ -298,7 +298,7 @@
       }
     }
     LOG(FATAL) << "Unreachable";
-    return StackMap(MemoryRegion());
+    UNREACHABLE();
   }
 
  private: