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 :white_check_mark:

Project coverage is 92.52%. Comparing base (c358484) to head (4d0f99e). Report is 78 commits behind head on main.

Additional details and impacted files

```diff @@ 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 ```

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