gc: fix msvc not using libatomic_ops (#15418)

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

@@ -0,0 +1,32 @@
+/*
+ * 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.
+ */
+
+#if defined(__clang__) || defined(AO_PREFER_BUILTIN_ATOMICS)
+  /* All __GCC_HAVE_SYNC_COMPARE_AND_SWAP_n macros are still missing.   */
+  /* The operations are lock-free even for the types smaller than word. */
+# define AO_GCC_FORCE_HAVE_CAS
+#else
+
+  /* As of gcc-7.5, CAS and arithmetic atomic operations for char and   */
+  /* short are supported by the compiler but require -latomic flag.     */
+# if !defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1)
+#   define AO_NO_char_ARITHM
+# endif
+# if !defined(__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2)
+#   define AO_NO_short_ARITHM
+# endif
+#endif /* !__clang__ */
+
+#include "generic.h"
+
+#undef AO_GCC_FORCE_HAVE_CAS
+#undef AO_NO_char_ARITHM
+#undef AO_NO_short_ARITHM