HIP: Default CMAKE_HIP_ARCHITECTURES when rocm_agent_enumerator fails

Currently we default CMAKE_HIP_ARCHITECTURES based on rocm_agent_enumerator, but if the enumeration fails, we fall back to parsing -target-cpu flags from the compiler identification step's output. This makes results unpredictable for projects, and currently we provide no way to distinguish the cases.

This was raised originally on discourse.

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