serpent-avx512-x86: fix CBC and CFB decryption with clang-18
* cipher/serpent-avx512-x86.c (serpent_avx512_blk32): Avoid '_mm512_castsi128_si512' usage to prevent non-initialized vector register parts getting XOR into calculations for CBC and CFB decryption.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>