crypto: x86/curve25519 - use in/out register constraints more precisely

Rather than passing all variables as modified, pass ones that are only
read into that parameter. This helps with old gcc versions when
alternatives are additionally used, and lets gcc's codegen be a little
bit more efficient. This also syncs up with the latest Vale/EverCrypt
output.

Reported-by: Mathias Krause <minipli@grsecurity.net>
Cc: Aymeric Fromherz <aymeric.fromherz@inria.fr>
Link: https://lore.kernel.org/wireguard/1554725710.1290070.1639240504281.JavaMail.zimbra@inria.fr/
Link: https://github.com/project-everest/hacl-star/pull/501


Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Reviewed-by: Mathias Krause <minipli@grsecurity.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>