Unify sys-crate build scripts

[justsmth]

Description

This PR unifies the build scripts for aws-lc-sys and aws-lc-fips-sys into a single shared ./builder directory. Both sys-crate builder directories are now symlinks to ../builder, eliminating significant code duplication between the two crates.

Call-outs

• fips feature for aws-lc-sys: This will not used by aws-lc-rs, but aws-lc-sys now supports a "fips" feature that triggers a FIPS-module build of its source.
• Simplified bindgen dependency: bindgen is now a standard build dependency for aws-lc-fips-sys. (Like with aws-lc-sys, it's no longer conditionally included based on target platform.)
• Removed rust_wrapper from FIPS: The FIPS build no longer produces a separate rust_wrapper library; helper functions are now implemented in pure Rust
• Added EnvGuard: RAII helper for temporary environment variable modifications during builds.

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 92.62%. Comparing base (c358484) to head (eb428b9).
⚠️ Report is 338 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #995      +/-   ##
==========================================
- Coverage   95.80%   92.62%   -3.18%     
==========================================
  Files          61       71      +10     
  Lines        8143    10053    +1910     
  Branches        0    10053   +10053     
==========================================
+ Hits         7801     9312    +1511     
- Misses        342      450     +108     
- Partials        0      291     +291     

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