Add the GetActiveRuntime function back into the x64 version of syzyasan_rtl.

syzygy/agent/asan/rtl_impl.cc

Index: syzygy/agent/asan/rtl_impl.cc
diff --git a/syzygy/agent/asan/rtl_impl.cc b/syzygy/agent/asan/rtl_impl.cc
index 11bd113e7af4c1c29a679b70914ef707f5060616..807eef9d7a0b9aa0d522a4aad4ee55c99fb68330 100644
--- a/syzygy/agent/asan/rtl_impl.cc
+++ b/syzygy/agent/asan/rtl_impl.cc
@@ -221,12 +221,12 @@ void WINAPI asan_SetOnExceptionCallback(OnExceptionCallback callback) {
   }
 }
 
-#ifndef _WIN64
 // Unittesting seam.
 AsanRuntime* WINAPI asan_GetActiveRuntime() {
   return asan_runtime;
 }
 
+#ifndef _WIN64
 void __declspec(naked) asan_SetAllocationFilterFlag() {
   __asm {
     pushad