arm64: Allow userspace to be built with PAC and BTI

Add the WITH/WITHOUT_BRANCH_PROTECTION build flags. This can be used
to enable the use of pointer authentication (FEAT_PAuth) and branch
target identification (FEAT_BTI) in userspace.

The kernel already handles both of these is userspace, we just need
to enable it.

Leave disabled for a short period for this to settle before enabling.

Reviewed by: emaste
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D42596