[RFC] ML-KEM: Add AArch64 arithmetic backend (from s2n-bignum)

[hanno-becker]

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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The purpose of this PR is to demonstrate and gather feedback on one option for integrating an AArch64 arithmetic backend for mlkem-native into AWS-LC.

Alternative option: #2498

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Context: The ML-KEM implementation in AWS-LC is imported from mlkem-native. mlkem-native comes in a "C-only" version, but also offers AArch64 and x86_64 backends for (a) arithmetic, and (b) FIPS-202. Currently, only the "C-only" version is imported into AWS-LC.

This commit adds a custom AArch64 backend to AWS-LC. The backend is essentially the same as in mlkem-native, but its assembly sources are taken from s2n-bignum and its headers are written from scratch. The constant tables used in the backend are copied from mlkem-native.

Compared to extending the mlkem-native->AWS-LC importer (see #2498) to include mlkem-native's AArch64 backend, this approach sticks to s2n-bignum as the sole source of verified assembly. It also provides greater flexibility in maintaining and adjusting the backend, both the assembly and the headers. For example, the assembly may be optimized for Graviton cores in the future, or the dispatch in the metadata files adjusted; the latter will mostly be relevant as we integrate x86_64 assembly, for which we aim to use the same methodology.

s2n-bignum is partially re-imported from @jargh's development branch https://github.com/jargh/s2n-bignum-dev/tree/mlkem/, restricting to the ML-KEM related files. Keeping this PR as Draft until John's work is merged into main of s2n-bignum, and the updated s2n-bignum has been re-imported in a separate PR.

To avoid a symbol clash with s2n-bignum, the mlkem-native namespace is changed from mlkem to mlkem_native.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.85%. Comparing base (587cf97) to head (c16676e).

Additional details and impacted files

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- Coverage   78.86%   78.85%   -0.01%     
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  Files         640      640              
  Lines      109560   109560              
  Branches    15522    15521       -1     
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- Hits        86402    86394       -8     
- Misses      22461    22468       +7     
- Partials      697      698       +1     

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