Fix compilation for aarch64-unknown-uefi

[crawfxrd]

In consideration of Thelio Astra:

CMOS access uses port IO, which is primarily an x86 thing. So the main question is:

• Can CMOS RAM be accessed on ARM (in a no_std environment) by some other means?

If not, gate CMOS interface, used for coreboot CMOS options, to x86.

[crawfxrd]

Related thought to #126: Extract the System76-specific flashing logic to a separate UEFI driver/application, since I'm moving towards having everything call out to model-specific UEFI shell scripts. This might actually be needed because of how much assumptions there are currently about systems being x86, or even Intel-specific.