AT_HWCAP2 not always defined

[justsmth]

Issues:

Addresses: aws/aws-lc-rs#427

Description of changes:

• Allow fallback when AT_HWCAP2 is not defined (for older glibc).

  AT_HWCAP2 (since glibc 2.18)
         Further machine-dependent hints about processor
         capabilities.

Testing:

• I've not yet been able to setup an environment that replicates the failure seen downstream for arm-unknown-linux-gnueabihf:

  In file included from /cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.18.0/aws-lc/crypto/fipsmodule/bcm.c:81:
  /cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.18.0/aws-lc/crypto/fipsmodule/cpucap/cpu_arm_linux.c: In function 'aws_lc_0_18_0_OPENSSL_cpuid_setup':
  /cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.18.0/aws-lc/crypto/fipsmodule/cpucap/cpu_arm_linux.c:121:38: error: 'AT_HWCAP2' undeclared (first use in this function); did you mean 'AT_HWCAP'?
       unsigned long hwcap2 = getauxval(AT_HWCAP2);
                                        ^~~~~~~~~
                                        AT_HWCAP
  /cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.18.0/aws-lc/crypto/fipsmodule/cpucap/cpu_arm_linux.c:121:38: note: each undeclared identifier is reported only once for each function it appears in
  /cargo/registry/src/index.crates.io-6f17d22bba15001f/aws-lc-sys-0.18.0/aws-lc/crypto/fipsmodule/bcm.c: At top level:
  cc1: error: unrecognized command line option '-Wno-c11-extensions' [-Werror]
  cc1: all warnings being treated as errors
  ninja: build stopped: subcommand failed.

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 78.25%. Comparing base (ba94617) to head (abe2889).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1682      +/-   ##
==========================================
- Coverage   78.28%   78.25%   -0.04%     
==========================================
  Files         567      567              
  Lines       95237    95237              
  Branches    13671    13671              
==========================================
- Hits        74561    74525      -36     
- Misses      20082    20115      +33     
- Partials      594      597       +3     

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[skmcgrail]

Just to double check, should we handle this case for other platforms as well if applicable?

[justsmth]

Just to double check, should we handle this case for other platforms as well if applicable?

Good point. I updated the PR to also handle this case for FreeBSD and ppc64le.