bhyve: add basl support for pointers

Some ACPI tables like XSDT contain pointers to other ACPI tables. When
an ACPI table is loaded by qemu's loader, the address in the guest
memory is unknown. For that reason, the qemu loader supports patching
those pointers. Basl keeps track of all pointers and causes the qemu
loader to patch all pointers.

The qemu ACPI table loader is unsupport yet. However, in a future commit
bhyve will use dynamic ACPI table offsets based on the size and
alignment requirements of each ACPI table. Therefore, tracking ACPI
table pointer is required too.

Reviewed by: jhb
Approved by: manu (mentor)
MFC after: 2 weeks
Sponsored by: Beckhoff Automation GmbH & Co. KG
Differential Revision: https://reviews.freebsd.org/D36991