bhyve: Provide optional libfdt linking

The arm64 port currently does not support ACPI, it instead builds up an
FDT which is exported to the guest. This mechanism will not be used on
amd64 but isn't really arm64-specific either, so provide an opt-in
mechanism to link libfdt.

No functional change intended.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D40995