Enable double-SHA256-for-64-byte code on 32-bit x86 #13393

The SSE4 and AVX2 double-SHA256-for-64-byte input code from #13191 compiles fine on 32-bit x86 systems, but the autodetection logic in sha256.cpp doesn't enable it. Fix this.

Note that these instruction sets are only available on CPUs that support 64-bit mode as well, so it is only beneficial in the (perhaps unlikely) scenario where a 64-bit CPU is running a 32-bit Bitcoin Core binary.

<!--e57a25ab6845829454e8d69fc972939a-->Note to reviewers: This pull request conflicts with the following ones:

• #13386 (SHA256 implementations based on Intel SHA Extensions 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.

utACK 57ba401abcfe564a2c4d259e0f758401ed74616d. See #13386 (comment) though.

As we've discussed on IRC I'm skeptical about doing anything special for 32-bit x86 as architecture (besides basic support), but as the changes are minimal I don't have opposition to this either.

@laanwj Right; given that we're already compiling and linking the SSE4/AVX2 code for 32-bit but just not using them, this seems much preferable over the alternative (removing them from the build).

@sipa agree; utACK 57ba401abcfe564a2c4d259e0f758401ed74616d