crypto: x86/curve25519 - leave r12 as spare register

This updates to the newer register selection proved by HACL*, which
leads to a more compact instruction encoding, and saves around 100
cycles.
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>