libstdc++: Avoid vector casts while still avoiding PR90424

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

libstdc++-v3/ChangeLog:

	PR libstdc++/109822
	* include/experimental/bits/simd_builtin.h (_S_store): Rewrite
	to avoid casts to other vector types. Implement store as
	succession of power-of-2 sized memcpy to avoid PR90424.