Gate Linux specific code to fix compilation on AIX

[pgimalac]

Issues:

None, but I can open one if needed.

I'm trying to build a Rust binary which depends on the aws-lc-sys crate on IBM AIX (a Unix OS), this PR gates linux-specific symbols to fix compiler errors.

Description of changes:

• Guard getrandom logic with OPENSSL_LINUX (on top of OPENSSL_RAND_URANDOM), I believe this logic is only reachable on Linux
• Guard RNDGETENTCNT with OPENSSL_LINUX as the symbol is defined in linux/random.h

Call-outs:

I'm aware AIX is not a supported platform, but the changes required to make it compile are minimal, and I think it makes the code technically more correct. Let me know what you think.

Testing:

Ran tests locally, built the library on an AIX host.

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 78.10%. Comparing base (cf95ba0) to head (8d8ecff).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3265      +/-   ##
==========================================
- Coverage   78.28%   78.10%   -0.19%     
==========================================
  Files         689      689              
  Lines      123463   123460       -3     
  Branches    17200    17196       -4     
==========================================
- Hits        96659    96429     -230     
- Misses      25881    26110     +229     
+ Partials      923      921       -2     

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[torben-hansen]

If IBM AIX could just get getentropy such as IBM z/OS does it, that would be great.

[pgimalac]

@torben-hansen agreed 😄

Thanks for the reviews !