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.

(cherry picked from commit bb4f8f14)