[fix](compile) guard MD5 AVX2-only symbols under __AVX2__

[airborne12]

What problem does this PR solve?

Issue Number: N/A

Related PR: N/A

Problem Summary:

The constants MD5_A0/MD5_B0/MD5_C0/MD5_D0 and the helpers md5_num_blocks() and md5_pad_final_blocks() in be/src/util/md5.cpp are only referenced by the AVX2 multi-buffer code path (md5_multi_buffer_compute / md5_binary_batch_avx2). On non-AVX2 builds (e.g. macOS ARM64) the AVX2 block is #ifdef'd out, leaving these symbols orphaned and triggering compile errors under the project's strict warnings:

error: unused variable 'MD5_A0' [-Werror,-Wunused-const-variable]
error: unused variable 'MD5_B0' [-Werror,-Wunused-const-variable]
error: unused variable 'MD5_C0' [-Werror,-Wunused-const-variable]
error: unused variable 'MD5_D0' [-Werror,-Wunused-const-variable]
error: unused function 'md5_pad_final_blocks' [-Werror,-Wunused-function]

This PR moves those AVX2-only constants and helpers inside the existing #ifdef __AVX2__ block where they belong. MD5_DUMMY_INPUT and md5_to_hex stay outside the guard because the non-AVX2 fallback in md5_hex_batch still uses them. No behavior change on x86_64; restores build on ARM/non-AVX2 targets.

Release note

None

Check List (For Author)

• Test

  • No need to test or manual test. Explain why:
    • This is a refactor/code format and no logic has been changed.
    • Previous test can cover this change.

  Manual test: verified ./build.sh --be -j 20 now succeeds on macOS ARM64.

• Behavior changed:

  • No.

• Does this need documentation?

  • No.

Check List (For Reviewer who merge this PR)

• Confirm the release note
• Confirm test cases
• Confirm document
• Add branch pick label

[hello-stephen]

Thank you for your contribution to Apache Doris.
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Please clearly describe your PR:

1. What problem was fixed (it's best to include specific error reporting information). How it was fixed.
2. Which behaviors were modified. What was the previous behavior, what is it now, why was it modified, and what possible impacts might there be.
3. What features were added. Why was this function added?
4. Which code was refactored and why was this part of the code refactored?
5. Which functions were optimized and what is the difference before and after the optimization?

[airborne12]

run buildall

[eldenmoon]

LGTM

[hello-stephen]

skip buildall

[github-actions]

PR approved by at least one committer and no changes requested.

[github-actions]

PR approved by anyone and no changes requested.

[hello-stephen]

skip buildall