libstdc++: Fix -Wformat warning in std::string

libstdc++: Fix -Wformat warning in std::string

The __throw_out_of_range_fmt function works like fprintf and so the
arguments corresponding to %zu specifiers need to be size_t. The
std::basic_string<C,T,A>::size_type type is A::size_type which is not
necessarily size_t. Add explicit casts to avoid a -Wformat warning with
-Wsystem-headers.

libstdc++-v3/ChangeLog:

	* include/bits/basic_string.h (basic_string::_M_check): Cast
	size_type arguments to size_t.