gc: fix msvc not using libatomic_ops (#15418)

thirdparty/libatomic_ops/atomic_ops/sysdeps/gcc/m68k.h

@@ -0,0 +1,68 @@
+/*
+ * Copyright (c) 1991-1994 by Xerox Corporation.  All rights reserved.
+ * Copyright (c) 1996-1999 by Silicon Graphics.  All rights reserved.
+ * Copyright (c) 1999-2003 by Hewlett-Packard Company. All rights reserved.
+ *
+ *
+ * THIS MATERIAL IS PROVIDED AS IS, WITH ABSOLUTELY NO WARRANTY EXPRESSED
+ * OR IMPLIED.  ANY USE IS AT YOUR OWN RISK.
+ *
+ * Permission is hereby granted to use or copy this program
+ * for any purpose,  provided the above notices are retained on all copies.
+ * Permission to modify the code and to distribute modified code is granted,
+ * provided the above notices are retained, and a notice that the code was
+ * modified is included with the above copyright notice.
+ *
+ */
+
+/* The cas instruction causes an emulation trap for the */
+/* 060 with a misaligned pointer, so let's avoid this.  */
+#undef AO_t
+typedef unsigned long AO_t __attribute__((__aligned__(4)));
+
+/* FIXME.  Very incomplete.  */
+#include "../all_aligned_atomic_load_store.h"
+
+/* Are there any m68k multiprocessors still around?     */
+/* AFAIK, Alliants were sequentially consistent.        */
+#include "../ordered.h"
+
+#include "../test_and_set_t_is_char.h"
+
+AO_INLINE AO_TS_VAL_t
+AO_test_and_set_full(volatile AO_TS_t *addr) {
+  AO_TS_t oldval;
+
+  /* The value at addr is semi-phony.   */
+  /* 'tas' sets bit 7 while the return  */
+  /* value pretends all bits were set,  */
+  /* which at least matches AO_TS_SET.  */
+  __asm__ __volatile__(
+                "tas %1; sne %0"
+                : "=d" (oldval), "=m" (*addr)
+                : "m" (*addr)
+                : "memory");
+  /* This cast works due to the above.  */
+  return (AO_TS_VAL_t)oldval;
+}
+#define AO_HAVE_test_and_set_full
+
+/* Returns nonzero if the comparison succeeded. */
+AO_INLINE int
+AO_compare_and_swap_full(volatile AO_t *addr,
+                         AO_t old, AO_t new_val)
+{
+  char result;
+
+  __asm__ __volatile__(
+                "cas.l %3,%4,%1; seq %0"
+                : "=d" (result), "=m" (*addr)
+                : "m" (*addr), "d" (old), "d" (new_val)
+                : "memory");
+  return -result;
+}
+#define AO_HAVE_compare_and_swap_full
+
+/* TODO: implement AO_fetch_compare_and_swap.   */
+
+#define AO_T_IS_INT