crypto: x86/chacha20 - Manually align stack buffer

The kernel on x86-64 cannot use gcc attribute align to align to
a 16-byte boundary.  This patch reverts to the old way of aligning
it by hand.

Fixes: 9ae433bc ("crypto: chacha20 - convert generic and...")
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>