s2n-bignum: Add prefix header to _s2n_bignum_internal.h

[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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AWS-LC's prefix build works by providing a 'prefix header' which prefixes global symbols through a series of #define ... directives. This header needs to be included ahead of the source files it applies to.

1. For C files, the prefix header is pulled in through openssl/base.h.
2. For perl-generated ASM files, it comes from openssl/asm_base.h which is #included through xxx-xlate.pl.
3. In contrast to 1. and 2., for s2n-bignum assembly, the prefix header is indirectly included through an --include directive in the CMakeLists.txt. Similarly, the cc_builder in aws-lc-rs is patched to use such --include clause when building s2n-bignum files.

This commit simplifies and improves the transparency of prefix handling for s2n-bignum files (case 3): Instead of using an --include directive in the build files, it explicitly adds the prefix header in the header _s2n_bignum_internal.h. This header is included by all s2n-bignum assembly. This is functionally equivalent to the previous --include directive, but avoids having to patch CMakeLists.txt or cc_builder.

The cc_builder patch can be removed from aws-lc-rs when convenient -- this commit renders the --include directive redundant, but not harmful.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.85%. Comparing base (9db959e) to head (cb0c2c4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2510      +/-   ##
==========================================
- Coverage   78.86%   78.85%   -0.01%     
==========================================
  Files         640      640              
  Lines      109560   109560              
  Branches    15521    15520       -1     
==========================================
- Hits        86401    86392       -9     
- Misses      22460    22469       +9     
  Partials      699      699              

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