libstdc++: Avoid ill-formed types on ARM

This resolves failing tests in check-simd.

Signed-off-by: Matthias Kretz <m.kretz@gsi.de>

libstdc++-v3/ChangeLog:

	PR libstdc++/114750
	* include/experimental/bits/simd_builtin.h
	(_SimdImplBuiltin::_S_load, _S_store): Fall back to copying
	scalars if the memory type cannot be vectorized for the target.