arm64: Clean up enabling in-kernel BTI

[andrew]

Summary

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.

Sponsored by: Arm Ltd

[markj]

sys/arm64/arm64/pmap.c

185–186

For consistency with other uses of the __read_mostly annotation.

[alc]

sys/arm64/arm64/locore.S

500

Typo fix.