Filter s2n-bignum symbols with prefix p256_montjscalarmul

[torben-hansen]

Description of changes:

To land aws/aws-lc#1877, must filter new s2n-bignum symbols that are not supposed to have external linkage. Similar to e.g. #286

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 92.52%. Comparing base (c358484) to head (782c108).
Report is 83 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #540      +/-   ##
==========================================
- Coverage   95.80%   92.52%   -3.28%     
==========================================
  Files          61       67       +6     
  Lines        8143     9277    +1134     
  Branches        0     9277    +9277     
==========================================
+ Hits         7801     8584     +783     
- Misses        342      422      +80     
- Partials        0      271     +271     

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