This avoids a bug in our outdated GNU as 2.17.50 that prevents linking with LLVM's lld linker, and almost all of amd64's assembly files are built using the compiler driver and IAS.
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D6789 / rS303158 addressed this for libmd by adding AFLAGS, but did not address modules/crypto. Although we could fix modules/crypto the same way I'd rather we use IAS everywhere.