alignas(16) unsupported w/ GCC 7.2 for ARM32 (#2086)

### 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.

crypto/cipher_extra/aead_test.cc

@@ -831,11 +831,17 @@ TEST_P(PerAEADTest, AliasedBuffers) {
   EXPECT_EQ(Bytes(kPlaintext), Bytes(in, out_len));
 }
 
+#if defined(__BIGGEST_ALIGNMENT__)
+#define UNALIGNED_TEST_ALIGNMENT __BIGGEST_ALIGNMENT__
+#else
+#define UNALIGNED_TEST_ALIGNMENT 8
+#endif // defined(__BIGGEST_ALIGNMENT__)
+
 TEST_P(PerAEADTest, UnalignedInput) {
-  alignas(16) uint8_t key[EVP_AEAD_MAX_KEY_LENGTH + 1];
-  alignas(16) uint8_t nonce[EVP_AEAD_MAX_NONCE_LENGTH + 1];
-  alignas(16) uint8_t plaintext[32 + 1];
-  alignas(16) uint8_t ad[32 + 1];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t key[EVP_AEAD_MAX_KEY_LENGTH + 1];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t nonce[EVP_AEAD_MAX_NONCE_LENGTH + 1];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t plaintext[32 + 1];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t ad[32 + 1];
   OPENSSL_memset(key, 'K', sizeof(key));
   OPENSSL_memset(nonce, 'N', sizeof(nonce));
   OPENSSL_memset(plaintext, 'P', sizeof(plaintext));
@@ -854,15 +860,15 @@ TEST_P(PerAEADTest, UnalignedInput) {
   ASSERT_TRUE(EVP_AEAD_CTX_init_with_direction(
       ctx.get(), aead(), key + 1, key_len, EVP_AEAD_DEFAULT_TAG_LENGTH,
       evp_aead_seal));
-  alignas(16) uint8_t ciphertext[sizeof(plaintext) + EVP_AEAD_MAX_OVERHEAD];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t ciphertext[sizeof(plaintext) + EVP_AEAD_MAX_OVERHEAD];
   size_t ciphertext_len;
   ASSERT_TRUE(EVP_AEAD_CTX_seal(ctx.get(), ciphertext + 1, &ciphertext_len,
                                 sizeof(ciphertext) - 1, nonce + 1, nonce_len,
                                 plaintext + 1, sizeof(plaintext) - 1, ad + 1,
                                 ad_len));
 
   // It must successfully decrypt.
-  alignas(16) uint8_t out[sizeof(ciphertext)];
+  alignas(UNALIGNED_TEST_ALIGNMENT) uint8_t out[sizeof(ciphertext)];
   ctx.Reset();
   ASSERT_TRUE(EVP_AEAD_CTX_init_with_direction(
       ctx.get(), aead(), key + 1, key_len, EVP_AEAD_DEFAULT_TAG_LENGTH,