libstdc++: Allow std::to_chars for 128-bit integers in strict mode

libstdc++: Allow std::to_chars for 128-bit integers in strict mode

This allows std::format to support __int128 when __STRICT_ANSI__ is
defined, which previously failed because __int128 is not an integral
type in strict mode.

With these changes, std::to_chars still rejects 128-bit integers in
strict mode, but std::format will be able to use __detail::__to_chars_i
for unsigned __int128.

libstdc++-v3/ChangeLog:

	* include/bits/charconv.h (__integer_to_chars_is_unsigned):
	New variable template.
	(__to_chars_len, __to_chars_10_impl): Use variable template in
	assertions to allow unsigned __int128 in strict mode.
	* include/std/charconv (__to_chars, __to_chars_16)
	(__to_chars_10, __to_chars_8, __to_chars_2): Likewise.