Remove special s2n-bignum symbol handling sauce from build

[torben-hansen]

Description of changes:

Symbols from s2n-bignum needed special build shenanigans because they essentially are "third-party" (although still AWS owned) in the context of what code AWS-LC repository can touch. Therefore they didn't have the special "prefix" header files. This corrupts the prefix build in two ways that needed to be handled:

• references to s2n-bignum files are prefixed, but the definitions are not. Obviously, this doesn't work...
• when we test the prefix build, we must take care to not assume s2n-bignum symbols are prefixed...

Removing these two corruptions, means it's now a bit easier to import s2n-bignum code. A small step closer to maybe switching to a submodule.

Should also make the rust build easier. Hopefully...

Call-outs:

Originally I went with a method that required s2n-bignum source code changes. Essentially, one can define a path to prefix symbols and inject into the pre-processor we run s2n-bignum files through. The benefit is that we don't have to reason about tooling. It's just simple macro gymnastics.

Andrew suggested --include flag as a way to not do s2n-bignum code changes. That seems a bit easier for now. We can revert to another more portable method if required. Having more AWS-LC special sauce in s2n-bignum is not the preferred for sure.

Testing:

CI has several test suites for the prefix build.

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 78.46%. Comparing base (ff56e55) to head (8199465).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1903   +/-   ##
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  Coverage   78.45%   78.46%           
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  Files         585      585           
  Lines       99517    99517           
  Branches    14241    14243    +2     
=======================================
+ Hits        78077    78084    +7     
+ Misses      20806    20797    -9     
- Partials      634      636    +2     

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