libstdc++: Fix -Wsign-compare warning in <charconv>

libstdc++: Fix -Wsign-compare warning in <charconv>

Cast ptrdiff_t to size_t to avoid a -Wsign-compare warning. We can check
in __to_chars_i that the ptrdiff_t won't be negative, so that we know
the cast is safe.

libstdc++-v3/ChangeLog:

	* include/std/charconv (__to_chars_16, __to_chars_10)
	(__to_chars_8, __to_chars_2, __to_chars): Cast ptrdiff_t to
	size_t for comparison.
	(__to_chars_i): Check for first >= last instead of first == last
	for initial sanity check.