Provide a replacement assembly implementation for SHA-512 on amd64.

Replace the perlasm generated SHA-512 assembly with a more readable
version and the same C wrapper introduced for SHA-256. As for

Provide a replacement assembly implementation for SHA-512 on amd64.

Replace the perlasm generated SHA-512 assembly with a more readable
version and the same C wrapper introduced for SHA-256. As for SHA-256,
on a modern CPU the performance is largely the same.

ok tb@

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