AWS-LC/BoringSSL macros and trycompiles can probably be cleaned up

[maddeleine]

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Problem:

#4572 introduced the try-compile s2n_libcrypto_supports_evp_aead_tls. This is probably the same thing as our macro S2N_AEAD_AES_GCM_AVAILABLE and potentially can be merged together, but its sort of unknown how large this change is. We have to look carefully at our macros to make sure they all mean the same thing.

Solution:

Look through the code and see where #if defined(OPENSSL_IS_BORINGSSL) || defined(OPENSSL_IS_AWSLC) is used. If it's code thats being used to gate aead logic, we can probably clean this all up to use only the libcrypto_supports macro.