Zig: implement self-hosted CPU model and features detection

[andrewrk]

Currently we rely on LLVM to do this:

https://github.com/ziglang/zig/blob/48cef8d5be24c40b4c845f6b07c78abd6c3d1b0d/lib/std/zig/system.zig#L790-L793

https://github.com/ziglang/zig/blob/48cef8d5be24c40b4c845f6b07c78abd6c3d1b0d/src-self-hosted/stage2.zig#L1156-L1167

The LLVM functions we are currently calling probably have some helpful code to look at when doing this implementation.

LLVM getHostCPUFeatures

[andrewrk]

Thanks to @alichay the x86 and x86-64 parts of this are complete!

[alichay]

Does it make sense to have the arch-avr label? While that's a valid target, of course, I can't see a situation where it makes sense for the compiler to run natively on it.