Fix profiler access to the Dart stack pointer on ARM64.

runtime/vm/signal_handler_macos.cc

Index: runtime/vm/signal_handler_macos.cc
diff --git a/runtime/vm/signal_handler_macos.cc b/runtime/vm/signal_handler_macos.cc
index bfcc42cdb05875af89bedd642e3e160dfbeb61cb..75c9c099d8d09c21dacf2ce443eb5640de70f4df 100644
--- a/runtime/vm/signal_handler_macos.cc
+++ b/runtime/vm/signal_handler_macos.cc
@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/globals.h"
+#include "vm/instructions.h"
 #include "vm/simulator.h"
 #include "vm/signal_handler.h"
 #if defined(TARGET_OS_MACOS)
@@ -68,7 +69,7 @@ uintptr_t SignalHandler::GetCStackPointer(const mcontext_t& mcontext) {
 
 uintptr_t SignalHandler::GetDartStackPointer(const mcontext_t& mcontext) {
 #if defined(TARGET_ARCH_ARM64) && !defined(USING_SIMULATOR)
-  return static_cast<uintptr_t>(mcontext->__ss.__x[19]);
+  return static_cast<uintptr_t>(mcontext->__ss.__x[SPREG]);
 #else
   return GetCStackPointer(mcontext);
 #endif