liblua: Add loader.machine and loader.machine_arch properties

[kevans]

Summary

Provisioned from MACHINE/MACHINE_ARCH on the system, expose loader.machine and loader.machine_arch respectively.

These may be used to hide ACPI option on non-applicable archs.

[imp]

These are fine, but I wonder if it might not be better to have a MD module in the loader that exports attributes of the architecture.... If we use this for more than a few things, we should consider that path instead...

[kevans]

In D14446#302859, @imp wrote:

These are fine, but I wonder if it might not be better to have a MD module in the loader that exports attributes of the architecture.... If we use this for more than a few things, we should consider that path instead...

Right, I've not yet found any machdep things that I care about in lua-land (other than being able to discriminate based on machine/machine_arch =)), so I think this is a good path to go on for the moment and we can change course if we come up with others.