libstdc++: Make incrementable<__int128> satisfied in strict mode

libstdc++: Make incrementable<__int128> satisfied in strict mode

This adds specializations of std::incrementable_traits so that 128-bit
integers are always considered incrementable (and therefore usable with
std::ranges::iota_view) even when they don't satisfy std::integral.

libstdc++-v3/ChangeLog:

	* include/bits/iterator_concepts.h [__STRICT_ANSI__]
	(incrementable_traits<__int128>): Define specialization.
	(incrementable_traits<unsigned __int128>): Likewise.
	* testsuite/std/ranges/iota/96042.cc: Test iota_view with
	__int128.