Determine the need for mingw-related stubs at compile time rather than

using not very significant $target_os.

grub-core/kern/misc.c

@@ -1283,7 +1283,7 @@ grub_fatal (const char *fmt, ...)
   grub_abort ();
 }
 
-#if NEED_REGISTER_FRAME_INFO && !defined(GRUB_UTIL)
+#if (defined (__MINGW32__) || defined (__CYGWIN__)) && !defined(GRUB_UTIL)
 void __register_frame_info (void)
 {
 }
@@ -1291,6 +1291,13 @@ void __register_frame_info (void)
 void __deregister_frame_info (void)
 {
 }
+void ___chkstk_ms (void)
+{
+}
+
+void __chkstk_ms (void)
+{
+}
 #endif
 
 #if BOOT_TIME_STATS