fix issues with SBCL, 32-bit platforms, and SHA512
fix issues with SBCL, 32-bit platforms, and SHA512

We were attempting to bit-bash (SIMPLE-ARRAY (UNSIGNED-BYTE 64) (*)) on
32-bit platforms, and that doesn't work so well.  Rework the test for
using bit-bashing to avoid this situation.
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# On branch mdx-digest-superclass
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#	modified:   src/digests/digest.lisp
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glv2/ironclad,sharplispers/ironclad,froydnj/ironclad,eadmund/ironclad