arm64: Set the Guarded Page flag in the kernel

Now the kernel and modules are built with branch protection we can
enablethe Guarded Page flag in the page tables. This causes indirect
branches to a location without a correct landing pad instruction to
raise an exception.

This should help mitigate some attacks where a function pointer is
changed to point somewhere other than the start of the function,
however it doesn't stop an attacker pointing it to an unintended
function.

Reviewed by: alc, scottph (both earlier version), markj
Sponsored by: Arm Ltd
Sponsored by: The FreeBSD Foundation (earlier version)
Differential Revision: https://reviews.freebsd.org/D42080