[DRAFT] Support GCC 4.8 for aarch64

[justsmth]

Issues:

Addresses: aws/aws-lc-rs#1022

Description of changes:

• GCC v4.8 was the first to support aarch64, but it does not provide the __ARM_ARCH macro for this target.
• This change ensure that this macro is defined correctly when building for this target.

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 78.29%. Comparing base (1f0e7fc) to head (a0523b1).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2964      +/-   ##
==========================================
- Coverage   78.46%   78.29%   -0.18%     
==========================================
  Files         689      689              
  Lines      120818   120796      -22     
  Branches    16969    16966       -3     
==========================================
- Hits        94802    94572     -230     
- Misses      25123    25333     +210     
+ Partials      893      891       -2     

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