Add an assembly file aes-xts-avx-x86_64.S which contains a macro that
expands into AES-XTS implementations for x86_64 CPUs that support at
least AES-NI and AVX, optionally also taking advantage of VAES,
VPCLMULQDQ, and AVX512 or AVX10.

This patch doesn't expand the macro at all.  Later patches will do so,
adding each implementation individually so that the motivation and use
case for each individual implementation can be fully presented.

The file also provides a function aes_xts_encrypt_iv() which handles the
encryption of the IV (tweak), using AES-NI and AVX.
Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>