arm64: Clean up enabling in-kernel BTI

Some hypervisors incorrectly use the Guarded Page (GP) bit from the
last level page table as part of the output address. This causes them
to raise an address size exception as the calculated physical address
is too large.

Only set the GP bit in the page table when BTI is present.

Reviewed by: alc, kib, markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D49154