* cipher/mceliece6688128f.c (crypto_int64_negative_mask)
(crypto_int16_negative_mask, crypto_int32_negative_mask)
(crypto_uint64_signed_negative_mask, crypto_uint16_signed_negative_mask)
(crypto_uint32_signed_negative_mask, mask_nonzero, mask_leq, vec_cmov)
(layer, gen_e, gf_mul, same_mask, pk_gen): Generate masks with
const-time helper function.
* src/const-time.h (ct_u64_gen_mask, ct_u64_gen_inv_mask): New.