crypto: curve25519-x86_64 - Use XORL r32,32

x86_64 zero extends 32bit operations, so for 64bit operands,
XORL r32,r32 is functionally equal to XORL r64,r64, but avoids
a REX prefix byte when legacy registers are used.

Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Acked-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>