hardware: rename none optimization flags to dunno

[alebcay]

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Follow-up to #18759. This helps catch the case of building bottles when the CPU type is dunno (in this case, Hardware.oldest_cpu returns :dunno).

Without this, the flag -march=dunno is passed when using the cc shim, which really doesn't work for any architecture.

[MikeMcQuaid]

Makes sense, thanks @alebcay!