* Fix bug in the output of the ct_verify function

A bug in the CCA transformation was reported on the pqc-forum on 2020-12-10
https://groups.google.com/a/list.nist.gov/g/pqc-forum/c/kSUKzDNc5ME

It was fixed today in 669522db63.
This commit ports that fix to PQClean

* add note to SECURITY.md

* update upstream commit in META.yml

Makes Kyber-AVX run on MacOS (#251)

It happened in another place

Oussama Danba caught a bug in how we were handing a test related to
ciphertext encoding. Not a security issue as far as we can tell, but
certainly not the correct behavior.

More detail here: e0ab9525f1

This commit also switches ntruhrss701/avx2/poly_S3_inv to the
Bernstein--Yang code, also slightly simplifies the poly_Rq_to_S3
routine.

This reverts commit 99bed07f64.