crypto: x86/aes-xts - make the register aliases per-function

Since aes-xts-avx-x86_64.S contains multiple functions, move the
register aliases for the parameters and local variables of the XTS
update function into the macro that generates that function.  Then add
register aliases to aes_xts_encrypt_iv() to improve readability there.
This makes aes-xts-avx-x86_64.S consistent with the GCM assembly files.

No change in the generated code.

Signed-off-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>