libstdc++: Fix fwrite error parameter

The first parameter of fwrite should be the const char* __s which want
write to FILE *__file, rather than the FILE *__file write to the FILE
*__file.

libstdc++-v3/ChangeLog:

	* config/io/basic_file_stdio.cc (xwrite) [USE_STDIO_PURE]: Fix
	first argument.

libstdc++-v3/config/io/basic_file_stdio.cc

@@ -128,7 +128,7 @@ namespace
     for (;;)
       {
 #ifdef _GLIBCXX_USE_STDIO_PURE
-	const std::streamsize __ret = fwrite(__file, 1, __nleft, __file);
+	const std::streamsize __ret = fwrite(__s, 1, __nleft, __file);
 #else
 	const std::streamsize __ret = write(__fd, __s, __nleft);
 #endif