crypto: arm64/aes-neon-ctr - improve handling of single tail block

Instead of falling back to C code to do a memcpy of the output of the
last block, handle this in the asm code directly if possible, which is
the case if the entire input is longer than 16 bytes.

Cc: Nathan Huckleberry <nhuck@google.com>
Cc: Eric Biggers <ebiggers@google.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>