Fix AES key size for AES256 in ABI test (aws#1629)

### Issues:
Resolves V1401987253

### Description of changes: 
This key buffer is too small for the larger key sizes that the test
uses.

### Call-outs:
It might be worth adding a address sanitizer + ABI test dimension. So
far we haven't hit this before so I didn't add a new docker image with
everything installed for that.

### Testing:
Ran locally and manually reviewed the change for memory safety. 

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/fipsmodule/modes/gcm_test.cc

@@ -239,7 +239,7 @@ TEST(GCMTest, ABI) {
 
 #if defined(OPENSSL_AARCH64) && defined(HW_GCM)
   if (hwaes_capable() && gcm_pmull_capable()) {
-    static const uint8_t kKey[16] = {0};
+    static const uint8_t kKey[256/8] = {0};
     uint8_t iv[16] = {0};
 
     for (size_t key_bits = 128; key_bits <= 256; key_bits += 64) {