alignas(16) unsupported w/ GCC 7.2 for ARM32

[justsmth]

Issues:

• Addresses: CryptoAlg-2718

Description of changes:

• When targeting arm, GCC 7.2 claims to not support alignas(16) in C++ code.

...
.../arm-unknown-linux-musleabi/crypto/cipher_extra/aead_test.cc:835:54: error: requested alignment 16 is larger than 8 [-Werror=attributes]
   alignas(16) uint8_t key[EVP_AEAD_MAX_KEY_LENGTH + 1];
                                                      ^
/home/justsmth/workspace/CrossAWS-LC-FIPS/src/CrossAWS-LC-FIPS/build/private/buildroot/arm-unknown-linux-musleabi/crypto/cipher_extra/aead_test.cc:836:58: error: requested alignment 16 is larger than 8 [-Werror=attributes]
   alignas(16) uint8_t nonce[EVP_AEAD_MAX_NONCE_LENGTH + 1];
                                                          ^
/home/justsmth/workspace/CrossAWS-LC-FIPS/src/CrossAWS-LC-FIPS/build/private/buildroot/arm-unknown-linux-musleabi/crypto/cipher_extra/aead_test.cc:837:39: error: requested alignment 16 is larger than 8 [-Werror=attributes]
   alignas(16) uint8_t plaintext[32 + 1];
...

Call-outs:

• We also use alignas(16) a few places in C code, but the compiler doesn't complain about those.
• Compilation of AWS-LC FIPS with this same compiler/target still fails with this change, but for an apparently unrelated reason.

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 78.75%. Comparing base (6aa30a9) to head (5407140).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2086      +/-   ##
==========================================
- Coverage   78.75%   78.75%   -0.01%     
==========================================
  Files         598      598              
  Lines      103653   103653              
  Branches    14720    14720              
==========================================
- Hits        81634    81629       -5     
- Misses      21365    21371       +6     
+ Partials      654      653       -1     

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