[fix](be) Fix unused constant/function build errors in md5.cpp

[zwy991114]

What problem does this PR solve?

Issue Number: close #xxx

Related PR: #63484

Problem Summary:

The constants MD5_A0/B0/C0/D0 and the helper functions md5_num_blocks / md5_pad_final_blocks in be/src/util/md5.cpp are only referenced inside the #ifdef __AVX2__ multi-buffer code path. On builds without AVX2 support, the compiler reports -Wunused-const-variable and -Wunused-function, which are promoted to errors via -Werror and break the build.

Move these AVX2-only declarations under the same #ifdef __AVX2__ guard that protects their sole consumer. MD5_DUMMY_INPUT and md5_to_hex remain outside the guard since both code paths use them.

Release note

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Check List (For Author)

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• Behavior changed:

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• Does this need documentation?

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  • Yes.

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[zwy991114]

/review

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