Improve behavior `output` in `magma_opts`

[leonardt]

Currently, fault requires you to use magma_output to set the magma compile target, but this may be confusing to user who is using magma_opts to passthrough arguments to m.compile (where they might think they can pass output as one of those arguments). We should either catch this case and raise an error (require the user to use magma_output), or allow the user to override the default magma_output using magma_opts (in this case, we can error if the user sets both magma_output and inside magma_opts since we don't necessarily know which one to use).