rijndael: harden mask generation against branch optimization

* cipher/rijndael.c (do_setkey): Generate 'rcon' mask with const-time
helper.
(_gcry_aes_xts_crypt): Generate tweak mask with const-time helper.

Cherry-pick master commit of:
f6b7a40f23c62a45ec1285388a5336c5cf313222

• Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>