Add a rangeset to the arm64 pmap to describe which address space needs
the Branch Target Identification (BTI) Guard Page flag set in the page
On hardware that supports BTI the Guard Page flag tells the hardware
to raise an exception if the target of a BR* and BLR* instruction is
not an appropriate landing pad instruction.
To support this in userspace we need to know which address space
should be guarded. For this add a rangeset to the arm64 pmap when the
hardware supports BTI. The kernel can then use pmap_bti_set and
pmap_bti_clear mark and unmark which address space is guarded.
Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation