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 <[email protected]>
Signed-off-by: Jason A. Donenfeld <[email protected]>
Signed-off-by: Herbert Xu <[email protected]>