Improve gcc-4.8 support/testing

[justsmth]

Description of changes:

• Pre-generated assembly was not usable with certain older compilers/assemblers such as gcc-4.8.
  • This change injects preprocessor macros to guard the portions of ADX/AVX2/AVX-512 assembly that would break GCC-4.8, allowing GCC-4.8 to now use the pre-generated assembly.

Call-out

• The pre-generated assembly is still not usable on the oldest supported compilers (e.g., gcc-4.1)

Testing:

• Added tests that build using the pregenerated assembly code.

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 :white_check_mark:

Project coverage is 78.18%. Comparing base (3da2c4a) to head (3c3f957).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #1665 +/- ## ========================================== - Coverage 78.21% 78.18% -0.03% ========================================== Files 571 571 Lines 95405 95404 -1 Branches 13705 13703 -2 ========================================== - Hits 74621 74592 -29 - Misses 20173 20201 +28 Partials 611 611 ```

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