libstdc++: Fix NEON intrinsic types usage

libstdc++: Fix NEON intrinsic types usage

Intrinsics types for NEON differ from gnu::vector_size types now. This
requires explicit specializations for __intrinsic_type and a new
__is_intrinsic_type trait.

libstdc++-v3/ChangeLog:

	* include/experimental/bits/simd.h (__is_intrinsic_type): New
	internal type trait. Alias for __is_vector_type on x86.
	(_VectorTraitsImpl): Enable for __intrinsic_type in addition for
	__vector_type.
	(__intrin_bitcast): Allow casting to & from vector & intrinsic
	types.
	(__intrinsic_type): Explicitly specialize for NEON intrinsic
	vector types.