Add missing register clearing in to SHA-256 and SHA-512 assembly
* cipher/sha256-ssse3-amd64.S: Clear used XMM/YMM registers at return. * cipher/sha512-avx-amd64.S: Ditto. * cipher/sha512-avx2-bmi2-amd64.S: Ditto. * cipher/sha512-ssse3-amd64.S: Ditto.
- Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>