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.
# Please enter the commit message for your changes. Lines starting
# with '#' will be ignored, and an empty message aborts the commit.
# On branch mdx-digest-superclass
# Changes to be committed:
#   (use "git reset HEAD <file>..." to unstage)
#
#	modified:   src/digests/digest.lisp
#