[CHERRYPICK FIPS 5.x] Add AArch64 NEON runtime fallback for ML-KEM and ML-DSA

[samuel40791765]

On aarch64, the ML-KEM/ML-DSA native primitives called the NEON assembly unconditionally, with no runtime path to the C reference implementation. This was fine since NEON assembly is a mandatory part of the base aarch64 architecture.

However, FIPS requires us to include a way to test the C fallback implementation on relevant architectures. This adds a runtime NEON capability gate on every native entry point so that when NEON is unavailable (reachable via OPENSSL_armcap=0) execution falls back to the C reference path, mirroring the existing x86-64/AVX2 gate.

This was implemented with the shadow-header pattern already used for x86-64 ML-DSA (mldsa_x86_64_meta.h). Since upstream mlkem-native does not have this, I've created two similar files mlkem_aarch64_meta.h / mldsa_aarch64_meta.h which mirror the vendored aarch64 meta.h. This allows no edits to the vendored mlkem/ or mldsa/ trees.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

(cherry picked from commit aba09cb)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license and the ISC license.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.17%. Comparing base (6283365) to head (91224c8).

Additional details and impacted files

@@                     Coverage Diff                      @@
##           fips-2026-06-26-snapshot    #3358      +/-   ##
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- Coverage                     78.21%   78.17%   -0.05%     
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  Files                           693      693              
  Lines                        123900   123900              
  Branches                      17208    17216       +8     
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- Hits                          96910    96855      -55     
- Misses                        26072    26126      +54     
- Partials                        918      919       +1     

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