crypto: curve25519 - do not pollute dispatcher based on assembler

Since we're doing a static inline dispatch here, we normally branch
based on whether or not there's an arch implementation. That would have
been fine in general, except the crypto Makefile prior used to turn
things off -- despite the Kconfig -- resulting in us needing to also
hard code various assembler things into the dispatcher too. The horror!
Now that the assembler config options are done by Kconfig, we can get
rid of the inconsistency.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>