[rust] Comment out __builtin_available() use (rust-lang#21)

__builtin_available() pulls in __isOSVersionAtLeast() from
compiler-rt. Comment it out so we don't have to figure out how
to pull in this symbol...

llvm/lib/Support/Unix/Path.inc

@@ -1353,7 +1353,7 @@ namespace fs {
 /// implementation.
 std::error_code copy_file(const Twine &From, const Twine &To) {
   uint32_t Flag = COPYFILE_DATA;
-#if __has_builtin(__builtin_available) && defined(COPYFILE_CLONE)
+#if 0 && __has_builtin(__builtin_available) && defined(COPYFILE_CLONE)
   if (__builtin_available(macos 10.12, *)) {
     bool IsSymlink;
     if (std::error_code Error = is_symlink_file(From, IsSymlink))