crypto: x86/curve25519 - support assemblers with no adx support

Some older version of GAS do not support the ADX instructions, similarly
to how they also don't support AVX and such. This commit adds the same
build-time detection mechanisms we use for AVX and others for ADX, and
then makes sure that the curve25519 library dispatcher calls the right
functions.

Reported-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>