crypto: x86/aesni-xts - deduplicate aesni_xts_enc() and aesni_xts_dec()

Since aesni_xts_enc() and aesni_xts_dec() are very similar, generate
them from a macro that's passed an argument enc=1 or enc=0.  This
reduces the length of aesni-intel_asm.S by 112 lines while still
producing the exact same object file in both 32-bit and 64-bit mode.

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