bhyve: build XSDT table by basl

Building the XSDT 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/D36999

(cherry picked from commit 4a60470f053dd035df6023457c153d3d86bc7123)