Restore CI for riscv64gc-unknown-linux-gnu

[justsmth]

Description of changes:

• Fix the cross-compilation test for riscv64.

Call-outs:

• Added support for a AWS_LC_SYS_CFLAGS environment variable that allows custom compile flags to be provided by the environment.
  • This was needed as a way propagate the -Wno-string-compare flag from the environment.
  • A similar problem was solved this way in the AWS-LC CI

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 (4d0f99e).
Report is 78 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #507      +/-   ##
==========================================
- 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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.