Specifically, move IO-APIC, LPC and PIRQ routing code under amd64/.
Use ifdefs to conditionally compile related code in other files. In
particular, legacy PCI interrupt handling is now compiled only on amd64.
This is not too invasive, but suggestions for a more modular approach
would be appreciated.
I am not sure why qemu fwcfg handling is tied to LPC, and I suspect it
should be decoupled. In this commit I just apply an ifdef hammer, but
we will eventually want fwcfg on arm64 as well.
No functional change intended.