bhyve: Do not compile PCI passthrough support on arm64

Some required kernel functionality is not yet implemented.

For now this means that one cannot specify host PCI register values, but
that functionality is only used by amd64-specific device models for now.
Note that this limitation is rather artificial; it arises only because
pci_host_read_config() lives in pci_passthru.c.

Reviewed by: corvink, andrew, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41738