UPSTREAM: 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>
(cherry picked from commit dc7fc3a53ae158263196b1892b672aedf67796c5)
Bug: 152722841
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>
Change-Id: I2eb1402dee0b9c209856bce69c9e352689929267