bhyve: Move legacy PCI interrupt handling under amd64/

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.

Reviewed by: corvink, jhb
MFC after: 1 week
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40739

(cherry picked from commit 55c13f6e7a412cc4bd0ea3fc183cd7c5c2348f01)