libstdc++: Always treat __float128 as a floating-point type

libstdc++: Always treat __float128 as a floating-point type

Similar to the previous commit that made is_integral_v<__int128>
unconditionally true, this makes is_floating_point_v<__float128>
unconditionally true. With the new extended floating-point types in
C++23 (std::float64_t etc.) it seems unhelpful for is_floating_point_v
to be true for them, but not for __float128. Especially as it is true on
some targets, because __float128 is just a typedef for long double.

This change makes is_floating_point_v<__float128> true whenever the type
is defined, giving less surprising and more portable behaviour.

libstdc++-v3/ChangeLog:

	* include/bits/cpp_type_traits.h (__is_floating<__float128>):
	Do not depend on __STRICT_ANSI__.
	* include/bits/stl_algobase.h (__size_to_integer(__float128)):
	Likewise.
	* include/std/type_traits (__is_floating_point_helper<__float128>):
	Likewise.

Reviewed-by: Patrick Palka <ppalka@redhat.com>