refactor: SHA256AutoDetect dead stores #14847

A rewrite of the SHA256AutoDetect function with some macroing to conditionally declare variables only if they are actually going to be used.

This removes multiple static analyzer complaints about dead stores (variables given a value and then discarded without reading the value ever).

It is also a slight clean-up as the (void)var; hacks are no longer necessary.

This seems a lot uglier than the existing (void)var; statements.

Made it a bit less ugly. Will see about making it even less so..

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Conflicts

Reviewers, this pull request conflicts with the following ones:

• #13989 (add avx512 instrinsic by fingera)
• #13442 (Convert the 1-way SSE4 SHA256 code from asm to intrinsics by sipa)

If you consider this pull request important, please also help to review the conflicting pull requests. Ideally, start with the one that should be merged first.