Validate or define ARM HWCAP2_XXX macros

[justsmth]

Issues:

Addresses: aws/aws-lc-rs#681

Description of changes:

On some platforms ARM's HWCAP2_XXX macros might be defined in system header files, causing compilation failure due to their being redefined here.

.../aws-lc/crypto/fipsmodule/cpucap/cpu_arm_linux.h:35:9: error: 'HWCAP2_AES' redefined [-Werror]
     35 | #define HWCAP2_AES (1 << 0)
        |         ^~~~~~~~~~
  In file included from .../armv7ve-libreelec-linux-gnueabihf/sysroot/usr/include/bits/hwcap.h:53:9: note: this is the location of the previous definition
     53 | #define HWCAP2_AES              1
        |         ^~~~~~~~~~

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 78.97%. Comparing base (e7bd073) to head (b90400b).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2164   +/-   ##
=======================================
  Coverage   78.97%   78.97%           
=======================================
  Files         611      611           
  Lines      105748   105748           
  Branches    14973    14972    -1     
=======================================
+ Hits        83511    83512    +1     
+ Misses      21583    21582    -1     
  Partials      654      654           

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