bhyve: build FADT table by basl

Building the FADT table by basl will allow it to be loaded by qemu's
ACPI table loader in the future.

Reviewed by: jhb, markj (older version)
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36998