libstdc++: Fix condition for supported SIMD types on ARMv8

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

libstdc++-v3/ChangeLog:

	PR libstdc++/110050
	* include/experimental/bits/simd.h (__vectorized_sizeof): With
	__have_neon_a32 only single-precision float works (in addition
	to integers).