libstdc++: Fix SFINAE for __is_intrinsic_type on ARM

libstdc++: Fix SFINAE for __is_intrinsic_type on ARM

On ARM NEON doesn't support double, so __is_intrinsic_type_v<double,
whatever> should say false (instead of being ill-formed).

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>
libstdc++-v3/ChangeLog:

	PR libstdc++/109261
	* include/experimental/bits/simd.h (__intrinsic_type):
	Specialize __intrinsic_type<double, 8> and
	__intrinsic_type<double, 16> in any case, but provide the member
	type only with __aarch64__.

(cherry picked from commit aa8b363171a95b8f867a74f29c75f9577e9087e1)