Minor fixes to SHA assembly implementations

* cipher/Makefile.am: Correct 'sha256-avx*.S' to 'sha512-avx*.S'.
* cipher/sha1-ssse3-amd64.S: First line, correct filename.
* cipher/sha256-ssse3-amd64.S: Return correct stack burn depth.
* cipher/sha512-avx-amd64.S: Use 'vzeroall' to clear registers.
* cipher/sha512-avx2-bmi2-amd64.S: Ditto and return correct stack burn
depth.
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Signed-off-by: Jussi Kivilinna <jussi.kivilinna@iki.fi>