Correct default structure packing on Windows ARM64

See issue #5454 for details

include/begin_code.h

@@ -107,7 +107,7 @@
 #ifdef __BORLANDC__
 #pragma nopackwarning
 #endif
-#ifdef _M_X64
+#ifdef _WIN64
 /* Use 8-byte alignment on 64-bit architectures, so pointers are aligned */
 #pragma pack(push,8)
 #else