Fix compilation for `aarch64-unknown-uefi`

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.

See also: https://github.com/system76/firmware-open/issues/587