libstdc++: Implement std::philox_engine for targets without __int128

libstdc++: Implement std::philox_engine for targets without __int128

This moves the __detail::_Select_uint_least_t<N>::type class to
namespace scope and extends it with more 128-bit arithmetic operations,
implemented in terms of uint64_t.

Now std::philox_engine can use _Select_uint_least_t<w*2>::type instead
of __uint128_t, so that it works on targets without 128-bit integers.
This also means that targets that do support __uint128_t only use it
when actually necessary, so that we use uint64_t when generating a
32-bit result (e.g. with std::philox4x32).

libstdc++-v3/ChangeLog:

	* include/bits/random.h [!__SIZEOF_INT128__] (__rand_uint128):
	Refactor and rename _Select_uint_least_t<128>::type to a new
	class. Make all members constexpr. Add new member functions for
	additional arithmetic and bitwise operations, and comparisons.
	(__detail::_Select_uint_least_t<>::type): Define as an alias of
	__rand_uint128.
	* include/bits/random.tcc (philox_engine::_M_mulhi): Use
	_Select_uint_least_t<w*2>::type instead of __uint128_t.
	(philox_engine::_M_transition): Likewise.
	* include/bits/version.def (philox_engine): Remove extra_cond.
	* include/bits/version.h: Regenerate.
	* testsuite/26_numerics/random/philox4x32.cc: Remove
	dg-require-cpp-feature-test directive.
	* testsuite/26_numerics/random/philox4x64.cc: Likewise.
	* testsuite/26_numerics/random/philox_engine/cons/copy.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/cons/default.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/cons/seed.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/operators/equal.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/operators/serialize.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/requirements/constants.cc:
	Likewise.
	* testsuite/26_numerics/random/philox_engine/requirements/typedefs.cc:
	Likewise.

Co-authored-by: Jakub Jelinek <jakub@redhat.com>
Reviewed-by: Tomasz Kamiński <tkaminsk@redhat.com>